Method and apparatus for purifying a gas containing contaminants

ABSTRACT

A method and an apparatus for purifying a gas containing contaminants are disclosed. The gas is irradiated with an ultraviolet ray and/or a radiation ray so as to produce microparticles of the contaminants. The resultant microparticles of the contaminants are contacted with a photocatalyst. Then, the photocatalyst is irradiated with light so as to decompose the contaminants held in contact with the photocatalyst. Organic compounds organosilicon compounds, basic gas and the like can be decomposed by the action of the photocatalyst. Even when these species are present at a low concentration, they can be concentrated locally by transforming into microparticles, and hence can be removed.

BACKGROUND OF THE INVENTION

The disclosures of the Japanese Patent Applications Nos. Hei-8-235832filed on the 20 Aug., 1996 and Hei-9-31441 filed on the 31 Jan., 1997are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for purifyinga gas containing contaminants. More specifically, the present inventionrelates to a method and an apparatus for purifying a gas by producingmicroparticles of the contaminants present in a gas and decomposing theresultant microparticles of contaminants with a photocatalyst forfacilitating the removal thereof.

RELATED ART

It was considered satisfactory in semiconductor industries in the pastto remove only solid particles such as dust from a gas such as air in aclean room. Methods for removing solid particles can be classifiedbroadly into 2 categories: (1) mechanical filtration methods (e.g. HEPA(High Efficiency Particulate Air) filter); and (2) methods for trappingmicroparticles electrostatically (e.g. MESA filter). Methods included inthe category (2) comprise charging microparticles electrically with ahigh electrical voltage and filtering the charged microparticles with anelectrically conductive filter. Gaseous contaminants, however, cannot beremoved by any method of either category.

Development of semiconductors of higher quality and finer precision hasmade it necessary to remove not only dust-like solid particles but alsogaseous contaminants Gaseous contaminants include: organic compoundsincluding phthalic esters; organosilicon compounds including siloxane;acidic gases including sulfur oxides (SOx), nitrogen oxides (NOx),hydrogen chloride (HCl) and hydrogen fluoride (HF); as well as basicgases including NH₃ and amines. Amines may be included among organiccompounds also. Anions such as NO₃ ⁻, NO₂ ⁻, SO₄ ²⁻, etc. havecharacteristics and exert adverse effects similar to acidic gases, andtherefor, are considered as a member of acidic gases out of convenience.Likewise, cations such as NH₄ ⁺, etc. have characteristics and exertadverse effects similar to basic gases, and therefor, are considered asa member of basic gases for convenience.

Organic compounds or organosilicon compounds, when deposited onto thesurface of a wafer (substrate), may have a negative effect on theaffinity (drapability) of a substrate for a resist. Decreased affinitymay exert a harmful influence on both the film thickness of a resist andthe adhesion of a substrate to a resist (“Air Cleaning”, Vol. 33, No. 1,pp. 16-21, 1995). For example, SOx may bring about defective insulationin an oxide layer. NH₃ may produce ammonium salts that are responsiblefor the blooming (poor resolution) of a wafer (Realize Inc., “SaishinGijyutsu Koza, Shiryo-shu”, 29 Oct. 1996, pp. 15-25, 1996). For theaforementioned reasons, such gaseous contaminants may diminish theproductivity (yield) of semiconductor products.

It was also considered satisfactory in the past to remove gaseouscontaminants to a level of ppm. It has become required now to removegaseous contaminants to a level of ppb. Among organic compounds, alkanessuch as methane and the like are not so reactive as to exert anunfavorable influence on a semiconductor, and hence are not required tobe removed to a level of ppb.

Removal of contaminants including organic compounds, especially gaseousorganic compounds is described below in more detail.

Known methods for removing organic compounds include decomposition bycombustion, catalytic decomposition, removal by adsorption,decomposition with O₃ and the like. These known methods, however, arenot effective in removing organic compounds present in lowconcentrations in air for feeding a clean room.

In a clean room, contamination with organic compounds of an extremelyslight concentration cannot be ignored. External organic compounds maybe introduced into a clean room. For example, outdoor air iscontaminated with organic compounds originating from exhaust gas of carsor those resulting from degassing of polymer products. On the otherhand, internal organic compounds may be generated in a clean room. Forexample, polymer materials (e.g. polymeric plasticizers, releasers,antioxidants and the like) which are used for constructing a clean roomare producers of organic gases (“Air Cleaning”, Vol. 33, No. 1, pp.16-21, 1995). Synthetic polymers are used in packing materials,sealants, adhesives and wall-forming materials in a clean room. Inaddition, plastic containers are disposed in a clean room. Thesesynthetic polymers may evolve a trace amount of organic gases. Moreparticularly, sealants and the production units thereof may give offgaseous siloxane, and plastic containers may give off gaseous phthalicesters. It has recently been found that gas evolves also from polymermaterials employed in a production unit. A process unit is partially orentirely surrounded by plastic plates which also produce organic gas. Avariety of solvents (e.g. alcohols, ketones, etc., which are necessaryfor operations in a clean room are also a contamination source.

As stated above, a clean room is contaminated variously and heavily withnot only organic compounds attributable to external air but also withorganic compounds and organosilicon compounds that are generatedinternally.

In view of energy saving considerations, recycling of air in a cleanroom has become more frequent recently. In consequence, organic gasesare progressively concentrated in a clean room, leading to heaviercontamination of the base materials of a wafer and a substrate. Theseorganic compounds are likely to deposit onto the bodies (e.g., startingmaterials and semi-fabricated products of a semiconductor wafer, a glasssubstrate, etc.) placed in a clean room, adversely affecting them.

A contact angle indicates a degree of contamination on a wafer substratewith organic compounds and organosilicon compounds. The contact anglerefers to the angle formed by the water and the surface of a substratewhen the surface is wet with water. The surface of a substrate, whencovered with a hydrophobic (oily) substance, becomes morewater-repellent and less wettable, hence the contact angle of water onthe surface of a substrate becomes larger. In other words, when thecontact angle is larger, the degree of contamination is higher. On thecontrary, when the contact angle is smaller, the degree of contaminationis lower.

When a substrate is contaminated with organic compounds andorganosilicon compounds, its affinity (drapability) for a resistdecreases, imparting an unfavorable influence on the resist and the filmthickness or on the adhesion of the substrate to the resist, that mayresult in lower quality and a lower yield.

Techniques in the high-technology field have made remarkable progress inrealizing semiconductor devices of a maximal precision and a minimalsize. In consequence, it has become necessary for a clean room to befree from organic compounds normally present in the air of the levelthat had conventionally been able to be ignored (an extremely lowconcentration of the ppb level) [Preparatory Manuscripts for the 39thMeeting of the Applied Physical Society, p. 86 (1992, Spring); “AirCleaning”, Vol. 33, No. 1, pp. 16-21, (1995)], as well as gaseouscontaminants including SO₂, HF, NH₃ [“Ultra Clean Technology”, Vol. 6,pp. 29-35 (1994)]. Because, it has been revealed that the presence ofthese gaseous contaminants diminished remarkably the productivity(yield). The present invention is aiming to efficiently remove thesegaseous contaminants.

The present inventors have proposed a method for removing hydrocarbonspresent in a gas comprising the steps of: irradiating the gas with anultraviolet ray and/or a radiation ray so as to produce microparticlesfrom the hydrocarbon; and trapping the resultant hydrocarbonmicroparticles with a filter or charging the hydrocarbon microparticleselectrically with a photoelectron and trapping the resultant chargedmicroparticles (Laid Open Japanese Patent Application No. Hei-5-96125).A similar method can be applied also to noxious matter present in a gas(Laid Open Japanese Patent Application No. Hei-4-243517).

Using the methods mentioned above, however, trapped microparticlesbecome accumulated on the filter or in the part for trapping the chargedmicroparticles, thus requiring frequent changing of the filter or thetrapping part. Further, when the accumulated microparticles fall fromthe filter or from the trapping part, the fallen microparticles, even ifthey are in extremely small amounts, inadvertently contaminate a gas tobe purified. Therefore, it is considered preferable to decomposecontaminants than to remove them.

A conventional removing method is now described with reference to FIGS.16 and 17. As shown in FIG. 16, the air which is fed to a clean room 1in a recycled manner is composed of the external air that is fed via apipe 2 and is cleared of coarse particles through a prefilter 3 and theinternal air that is drawn out of the clean room 1 through an air outlet4. Both airs are combined in a fan 5, conditioned in temperature andmoisture with an air conditioner 6 and cleared of microparticles with aHEPA filter 7. The air in the clean room is kept at a purity (class) ofthe order of 10,000. In this specification, the term “class” refers tothe number of particles having a particle diameter of not less than 0.1μm that are present per cubic feet.

A clean bench 51 is disposed in the clean room 1 to trap and remove atrace amount of hydrocarbons and microparticles (particulate matter).

Organic compounds present in the clean room 1 may consist presumably ofthose that originating in external air introduced through the pipe 2(those that are presumably discharged from cars and synthetic resins)and those that are produced during operations in the clean room.

The clean bench 51 comprises mainly a microparticle-producing 48, amicroparticle-charging section 49 and a section for trapping chargedmicroparticles 50. A highly pure air (of class 10) that is freed of bothorganic compounds and coexistent microparticles is fed above a workingtable 53, where operations are being carried out.

In other words, air having a purity (class) in the order of 10,000 andcontaining a trace amount of organic compounds originating in the cleanroom 1 is directed with a fan (not shown) toward the clean bench 51. Atthe clean bench 51, the microparticle-producing section 48 is providedfor irradiating the air with an ultraviolet radiation of a shortwavelength so as to produce microparticles of organic compoundscontained in the air. Then, in the microparticle-charging section 49,the microparticles are electrically charged efficiently withphotoelectrons emitted by a photoelectron-emitting material as describedhereinbelow. The resultant charged microparticles are trapped andremoved in the section for trapping charged microparticles 50 thatfollows. In this manner, air above the working table 53 can bemaintained to be highly pure and free of organic compounds.

A movable shutter is provided on the clean bench 51 for facilitatingintroduction and/or withdrawal of instruments and products into and/orout of the working table 53.

FIG. 17 shows schematically a microparticle-producing section 48, amicroparticle-charging section 49 and a section for trapping chargedmicroparticles 50. These sections are described just below withreference to FIG. 17.

In other words, air 54 aspirated through a fan (not shown) andcontaining a trace amount of organic compounds is filtered through aprefilter (not shown), and then irradiated with an ultraviolet radiationof a short wavelength in the microparticle-producing section 48 that ismainly consisting of an UV lamp 55. Organic compounds present in the air54 are transformed into microparticles 56 by UV irradiation. Thesemicroparticles 56, together with naturally-occurring microparticles 57already present in the introduced air 54, are electrically charged inthe microparticle-charging section 49 so as to become chargedmicroparticles 58.

The microparticle-charging section 49 is mainly composed of an W lamp59, a photoelectron-emitting material 60 (herein consisting of a glassmaterial having a surface coated with an Au thin layer of a thickness of5 to 50 nm, for example) and an electrode material 61 for generating anelectrical field. The photoelectron-emitting material 60 is irradiatedwith the UV lamp 59 in the presence of an electrical field so as to emitphotoelectrons 62, which in turn supply the microparticles 56, 57 withan electrical charge so as to produce the charged microparticles 58,which can then be trapped in the section for trapping chargedmicroparticles 50 that follows. The section 50 consists of a materialfor trapping the charged microparticles. Reference numeral 63 indicatesan V-transmissive material. Reference numeral 64 indicates a highly pureair that is dust-free and free from organic compounds.

The arrangement as stated above is suffered from the problems as setforth below:

(1) Microparticles that were produced from organic compounds uponirradiation with an ultraviolet ray and/or a radiation ray often failedto result in complete trapping with a filter or complete charging andtrapping with photoelectrons, depending on the irradiation conditionsand the kinds of organic compounds. This is probably because some kindsof organic compounds tend to produce microparticles of an extremelysmall size. Or else, the chemical composition of organic compounds maybe responsible for it. In case when the trapping efficiency was low, atrapping section having a larger volume was required, hence making thewhole apparatus larger.

(2) Produced particulate matter can be trapped at the trapping section50. Consequently, this particulate matter tends to accumulate in thetrapping section during a long-term continuous operation. This requiresa design of a trapping section 50 having a higher trapping volume, thusmaking the size of an apparatus larger.

On the other hand, the present inventors have proposed the use of aphotocatalyst in a system for removing organic compounds (JapanesePatent Applications Nos. Hei-8-31230 and Hei-8-31231). In this system,however, organic compounds in a low concentration are decomposed with aphotocatalyst so slowly that the decomposition thereof is verytime-consuming. In other words, diethylhexyl phthalate (DOP) andsiloxane present in the natural air and the air in a clean room are onlyin a concentration as low as about 1 ppb each.

Further, photocatalysts cannot effectively remove acidic gases such asS0 ₂, NO, HCl and HF. In particular, sulfur-containing compounds such assulfur oxides, hydrogen sulfide, thiophene and thiols, when present at ahigh concentration, may sometimes act as a catalytic poison to thephotocatalysts. Even if these compounds can avoid acting as a catalyticpoison, they might adversely influence on the photocatalysts after along-term operation.

SUMMARY OF THE INVENTION

The present invention is directed to solve the problems as set forthabove.

According to one aspect of the present invention, there is provided amethod for purifying a gas containing a contaminant comprising amicroparticle-producing step for irradiating the gas with an ultravioletray and/or a radiation ray so as to produce microparticles of thecontaminant, a contact step for contacting the microparticles of thecontaminant with a photocatalyst and a first decomposition step forirradiating the photocatalyst with a light so as to decompose thecontaminant being in contact with the photocatalyst. Organic compounds(except for alkanes), organosilicon compounds and basic gases can beoxidatively decomposed with a photocatalyst. Even when contaminants arepresent in small amounts, they can be concentrated locally bytransforming into microparticles, and hence can be oxidativelydecomposed with a photocatalyst efficiently.

In the microparticle-producing step, an ultraviolet ray and/or aradiation ray having a wavelength of not more than 260 nm is preferablyused. Contaminants can aggregate through a radical reaction to producemicroparticles.

Preferably, the gas contains water or gaseous oxygen in a concentrationof not less than 1 ppb. More preferably, the gas contains water orgaseous oxygen in a concentration of not less than 100 ppb. It isbelieved that water or gaseous oxygen acts on the surface of aphotocatalyst by supplying it with OH radical to induce activation ofthe photocatalyst. The OH radical probably acts as an oxidant in thepresence of the photocatalyst.

It is preferred that: a gas contains gaseous oxygen of at least 1 ppm;the gaseous oxygen present in the gas is transformed into ozone at themicroparticle-producing step; and the method further comprises a seconddecomposition step for decomposing the resultant ozone.

More preferably, the method comprises a removal step for removingcontaminants. Preferably, the contaminants contain acidic or basiccompounds, and more preferably, the contaminants contain at least onespecies selected from the group consisting of nitrogen oxides (NOx),nitrogen oxide ions, sulfur oxides (SOx), sulfur oxide ions, hydrogensulfide, hydrogen fluoride, ammonia and amines.

The removal step may precede the microparticle-producing step.Alternatively, the removal step may follow the microparticle-producingstep and precede the first decomposition step. The latter order issuitable when there is contained any contaminant serving as poison to aphotocatalyst. More precisely, when sulfur-containing compounds such assulfur oxides, hydrogen sulfide, thiophene and thiols are present, it ispreferred that these compounds are removed prior to the treatment withthe photocatalyst. Alternatively, the removal step may followimmediately after the first decomposition step.

Preferably, the removal step is carried out by means of one or more of afilter, an adsorbent, an ion exchanger and a photoelectron.Photoelectrons can supply contaminants with an electrical charge so asto facilitate the trapping of the contaminants.

Preferably, the photocatalyst is composed of a matrix and acatalytically active component which is carried on the matrix and whichis preferably in the form a particle. More preferably, the matrix is inthe form of a honeycomb structure provided with partitions defining atleast 2 through-holes, a bar body or a wall member, and thecatalytically active component is semiconductor.

According to the second aspect of the present invention, there isprovided an apparatus for purifying a gas containing a contaminantcomprising: a microparticle-producing section having a source of anultraviolet ray and/or a radiation ray; and a decomposition sectionhaving a photocatalyst and a light source for irradiating thephotocatalyst, the decomposition section being connected to themicroparticle-producing section.

Preferably, the apparatus is provided with a gas inlet and a gas outletand is disposed in a manner that the gas inlet, themicroparticle-producing section, the decomposition section and the gasoutlet are arranged successively downstream.

More preferably, an ozone-decomposing material is provided downstream tothe microparticle-producing section.

In addition, it is preferred to provide a removal section for removingacidic and/or basic compounds. Preferably, the removal section comprisesone or more means selected from a filter, an adsorbent and an ionexchanger as well as a photoelectron-emitting means and a means fortrapping charged contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of a clean room having a purification apparatusof the present invention.disposed therein.

FIG. 2 is a general view of a wafer stocker having a purificationapparatus of the present invention disposed therein.

FIG. 3 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 4 is a graph of the contact angle (degrees) versus storage period(days) showing the results of Example 4.

FIG. 5 is a general view of a clean room having a purification apparatusof the present invention disposed therein.

FIG. 6 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 7 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 8 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 9 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 10 is a general view of a wafer stocker having a purificationapparatus of the present invention disposed therein.

FIG. 11 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 12 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 13 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 14 is a cross-sectional view showing another embodiment of thepurification apparatus of the present invention.

FIG. 15 is a graph of the contact angle (degrees) versus storage period(days) showing the results of Example 14.

FIG. 16 is a general view of a conventional clean room.

FIG. 17 is an enlarged partial view of an air-purifying section of theapparatus shown in FIG. 16.

FIG. 18 is a general view of a wafer stocker applying the purificationmethod of the present invention.

FIG. 19 is a general view of a wafer stocker applying anotherpurification method of the present invention.

FIG. 20 is a general view of a wafer stocker applying anotherpurification method of the present invention.

FIG. 21 is a general view of a wafer stocker applying anotherpurification method of the present invention.

FIG. 22 is a schematic view showing the purification of air for feedingan air knife by applying another purification method of the presentinvention.

FIG. 23 is a graph showing the change of the contact angle (degrees) asthe function of time (days).

FIG. 24 is a graph showing the number of days taken for 5-degreeincrease in the contact angle as the function of SOx concentration.

FIG. 25 is a total ion chromatogram of hydrocarbons present in the airobtained by gas chromatography/mass spectrometry (GC/VS) method.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a gas containing contaminants ispurified. Contaminants which can be removed by the present invention aremainly gaseous contaminants including: organic compounds (except foralkanes such as methane); organosilicon compounds such as siloxane;acidic gases such as sulfur oxides (SOx), nitrogen oxides (NOx),hydrogen chloride (HCl) and hydrogen fluoride (HF); as well as basicgases such as NH₃ and amines.

Organic compounds include: aliphatic hydrocarbons, especially loweraliphatic hydrocarbons having 1 to 40 carbon atoms, such as alkene,alkyne and the like; aromatic hydrocarbons, especially lower aromatichydrocarbons having 1 to 40 carbon atoms, such as benzene, naphthaleneand the like; alcohols, especially lower alcohols having 1 to 40 carbonatoms; phenols; carboxylic acids such as higher fatty acids; carboxylicacid derivatives such as esters, amides, acid anhydrides and the like;ethers; amines; as well as sulfur-containing compounds such assulfoxides, mercaptons, thiols and the like. Examples of aromatichydrocarbons which can be removed by the present invention includebenzene, toluene, ethylbenzene and the like.

Carboxylic acid derivatives include phthalic esters such as butylphthalate originating in synthetic polymers the phthalic esters andhence can be removed by the present invention.

Nitrogen-containing heterocycles such as pyrrole and pyridine,oxygen-containing heterocycles such as furan and tetrahydrofuran, aswell as sulfur-containing heterocycles such as thiophene can be removedas well.

Halogenated hydrocarbons include: halogen-containing aliphatic compoundsincluding trihalomethane such as chloroform as well astrichlorofluoromethane, dichloromethane and dichloroethane;halogen-containing aromatic compounds such as chlorophenol.

On the other hand, contaminants do not include alkanes such as methaneand the like, since they are less reactive and are difficult to depositonto a substrate made of a semiconductor and the like. Hence, alkanesare not required to be removed by the present invention. Alkanes havingat most 4 carbon atoms are more difficult to deposit onto asemiconductor substrate and those having at most 3 carbon atoms arestill more difficult to deposit onto a semiconductor substrate.

The present invention will now be described in more detail section bysection.

According to the present invention, a gas containing contaminants suchas gaseous contaminants is irradiated with an ultraviolet ray and/or aradiation ray. An apparatus of the present invention-comprises amicroparticle-producing section having a source of an ultraviolet rayand/or a radiation ray.

The microparticle-producing section has an irradiation source fortransforming gaseous contaminants such as organic compounds present in agas into microparticles (condensable matter or particulate matter). Anyirradiation source can be used, provided that it allows organiccompounds and coexistent gaseous contaminants such as SO₂ and NH₃ totransform into microparticles or particulate matter. Any ofelectromagnetic waves, lasers, radiations can be used as desired inaddition to ultraviolet rays. Also, suitable irradiation sources can beselected on the basis of the results of preliminary experiments,depending on fields of application, compositions and concentrations ofthe gaseous contaminants to be removed, sizes and shapes of apparatusesas well as economical efficiency. Usually, irradiation with anultraviolet or a radiation ray is preferred.

Upon irradiation, the contaminants present in the gas are transformedinto particulate matter (condensable matter) optionally accompanied byactive matter (condensable matter), depending on the constituentsthereof and other matter coexistent therewith. For example, when agaseous mixture containing toluene, isopentane and propylene as organiccompounds is irradiated with an ultraviolet ray, carboxylic acids andcarbonyl compounds (condensable matter or active matter) are produced.

The presence of higher aliphatic acids having a high molecular weight,phenol derivatives, phthalic esters (e.g. DBP, DOP) and siloxane in aclean room has recently become a serious problem, since they areincluded among the organic compounds that deposit onto a substrateincluding wafer and induce an increase in the contact angle (“AirCleaning”, Vol. 33, No. 1, pp. 16-21, 1995). DBP is an abbreviation ofdibutyl phthalate. DOP is an abbreviation of dioctyl phthalate, which iscalled more accurately di-(2-ethylhexyl)phthalate. Phthalic esters suchas DBP, DOP and the like are useful as a plasticizer of resins, inparticular of vinyl resins.

These organic compounds and organosilicon compounds including siloxaneare transformed into particulate matter upon irradiation with anultraviolet ray and/or a radiation ray. The resultant particulate matterhas a particle diameter of e.g. scores of nanometers to several hundrednanometers. This may be probably because the contaminants exposed toultraviolet or other radiation undergo a radical reaction with gaseousoxygen and water present in a gas at a trace amount of the order of 1ppb or more, whereby the contaminants are brought into a state ofassociation.

According to the present invention, contaminants are transformed intomicroparticles as stated above. In other words, contaminants aremicroscopically concentrated. Then, the resultant microparticles ofcontaminants are decomposed with a photocatalyst.

By way of another example, the reaction schemes of S0 ₂ are set forthbelow:H₂O⁺+e⁻→.OH+H.H₂O⁺+H₂O→H₃O⁺+.OHSO₂+.OH→SO₃ ⁻SO₃+H₂O→H₂SO₄H₂SO₄→microparticles (particulate matter)

H₂O means the water contained in a gas at a trace amount. During thefinal reaction, microparticles are produced from sulfuric acid. It maybe also probable in this reaction that when a highly viscous liquid suchas sulfuric acid is produced in a gas even in only a small amount, othercontaminants can associate therewith to produce microparticles. Thepresence of a basic gas, e.g. ammonia may induce a reaction of acidicgases.

Another example of the reaction schemes is shown below:O₂ ⁺(UV)→O+OO₂+O→O₃O₃→O+O₂O+H₂O →2OHOH+SO₂+O₂→SO₃+HO₂SO₃+H₂O→H₂SO₄H₂SO₄+H₂O→microparticles (particulate matter)

These reactions vary depending on the kinds of coexistent gases,irradiation conditions and others. As shown in the above schemes, waterreacts with coexistent gaseous contaminants (SO₂ in this case) to afforda reaction product, which is SO₃ ⁻ in the above. By transforming thereaction product into microparticles followed by trapping and removingthe resultant microparticles, a gas is cleared of coexistent gaseouscontaminants such as SO₂ and becomes highly pure. This action is notlimited to SO₂, but is common to other various gaseous contaminants suchas NH₃ and the like.

Transformation into microparticles (production of microparticles) can beinduced effectively with an irradiation source having a wavelength ofnot more than 260 nm, preferably of not more than 254 nm. Usually,irradiation sources of ultraviolet and/or other radiation are preferredin view of effects and operability.

Any UV source can be used, provided that the irradiation therewith canproduce microparticles of organic compounds and those of coexistentgaseous contaminants including SO₂ and NH₃ (transformation intoparticulate matter or condensable matter or transformation intomicroparticles and active matter). Suitable U sources can be selected onthe basis of the results of preliminary experiments, depending on thekinds of organic compounds and other coexistent matter. UV sources thatproduce oxygen-activated species (active radicals) such as active oxygenand OH radical may be preferred depending on the field of application.

In general, suitable UV irradiation sources include a mercury lamp, ahydrogen discharge tube (heavy hydrogen lamp) and the like. It ispreferred to use an UV irradiation source having multiple wavelengthswhich can induce different actions depending on the kinds of organiccompounds and coexistent gaseous contaminants including SO₂, NH₃, etc.or coexistent matter. For example, a mercury lamp can use: (1) anozone-producing wavelength; in combination with (2) a wavelength forinducing decomposition of the resultant ozone so as to promote theproduction of oxygen-activated species. By way of an example, thesewavelengths are respectively of 184 nm and 254 nm. Production ofmicroparticles of organic compounds and coexistent gaseous contaminantsincluding SO₂ and NH₃ is mainly induced at 184 nm and decomposition ofthe produced ozone is mainly induced at 254 nm. Ozone is preferredbecause it has an ability to promote transformation of gaseouscontaminants into microparticles as shown in the reaction schemes setforth above.

Radiation rays which can be suitably used are α-ray, β-ray, γ-ray andthe like. Irradiation means which can be used are: a radiation sourceutilizing radioactive isotopes such as cobalt 60, cesium 137 andstrontium 90, radioactive wastes of a nuclear reactor or radioactivematerials obtained therefrom through suitable processing; a radiationsource utilizing directly a nuclear reactor; a radiation sourceutilizing a particle accelerator such as electron beam accelerator.Electron beam irradiation with an accelerator can become highly denseand effective by being applied at a low output power. An acceleratingvoltage is at most 500 kV, preferably in the range of 50 kV to 300 kV.

According to the present invention, contaminants in the form ofmicroparticles are made contact with a photocatalyst. Contact includesdeposition and adsorption. The present invention is intended for theremoval of contaminants having a tendency to increase the contact angleon the surface of a substrate such as a wafer. These contaminants arelikely to deposit not only onto the wafer surface but also onto thephotocatalyst. In addition, contaminants in the form of microparticlesmay deposit onto the photocatalyst through Brownian motion.

According to the method of the present invention, a photocatalyst isirradiated with light. In other words, the apparatus of the presentinvention comprises a decomposition section including a photocatalystand a light source for irradiating the photocatalyst.

Photocatalysts are now described. Photocatalysts are described in U.S.patent application Ser. No. 08/733,146, the disclosure of which isherein incorporated by reference.

Any photocatalyst can be used, provided that the excitation thereof byirradiation with light can promote an oxidative reaction. Photocatalystscan oxidatively decompose organic compounds, organosilicon compounds,basic gases such as ammonia. For example, organic compounds aredecomposed into low molecular non-toxic substances, such as carbondioxide and water. Organosilicon compounds are decomposed into carbondioxide and water. It is not completely elucidated-whether atomicsilicium is oxidatively decomposed to produce SiO₂. Ammonia is thoughtto be oxidatively decomposed into gaseous nitrogen.

Photocatalysts are not required to oxidatively decompose organiccompounds down to carbon dioxide. When a photocatalyst is used toprevent the increase in the contact angle on the surface of asemiconductor wafer, it is required only to transform organic compoundsinto the compounds playing no part in the increase in the contact angle,in other words, stable compounds having no adverse effects even if theyare deposited on the surface of the semiconductor wafer.

On the other hand, photocatalysts are thought to oxidize acidic gasessuch as sulfur oxides, nitrogen oxides and the like, too. By way of anexample, sulfur oxides (SOx) are oxidized into SO₂, which may possiblyreact with water in the air to produce sulfuric acid. Acidic gasestransformed into the form of microparticles are preferably removed withany of a filter, an adsorbent or an photoelectron.

Photocatalysts contain catalytically active components, which arepreferably semiconductors. Such semiconductors include: elementarysemiconductors such as Si, Ge and Se; compound semiconductors such asAlP, AlAs, GaP, AlSb, GaAs, InP, GaSb, InAs, InSb, CdS, CdSe, ZnS, MoS₂,WTe₂, Cr₂Te₃, MoTe, Cu₂S and WS₂; oxide semiconductors such as TiO₂,Bi₂O₃, CuO, Cu₂O, ZnO, MoO₃, InO₃, Ag₂O, PbO, SrTiO₃, BaTiO₃, Co₃O₄,Fe₂O₃, NiO, WO₃ and SnO₂. Preferable oxides are titanium oxide, titaniumstrontium trioxide, cadmium sulfide, zinc oxide, tungsten oxide and tinoxide, and, more preferable oxides are titanium oxide, titaniumstrontium trioxide and zinc oxide. Titanium oxides respectively ofrutile structure and anatase structure are useful.

Cocatalysts such as Pt, Ag, Pd, RuO₂, Co₃O₄ and the like can be added tocatalytically active components so as to improve the catalytic action ofphotocatalysts. Cocatalysts can be used alone or in combination of twoor more. Cocatalysts can be added by using any of suitable well knownmethods such as impregnation, photoreduction, sputtering evaporation,kneading and the like.

Catalytically active components are preferably formed in the shape of aparticle so as to increase the surface area thereof. When a cocatalystis used, individual particles are composed of a photocatalyst and acocatalyst.

A photocatalyst is preferably composed of a matrix and a catalyticallyactive component carried on the matrix. The catalytically activecomponent can be fixed to the matrix by being coated onto the surface ofthe matrix or by being wrapped in the matrix or inserted into thematrix. Matrices can be made of ceramics, fluororesins, glass, glassymaterials or various metals. A matrix may be formed in the shape of ahoneycomb, a wire cloth, a fiber, a rod and a filter. The term“honeycomb” as used herein means a structure provided with through-holesof any shape in cross-section. The cross-sectional shape of athrough-hole may be selected from a circle, an ellipse and a polygon,for example.

By way of an example, a matrix may be a honeycomb structure providedwith partitions defining two or more through-holes. A photocatalyst inthe form of particles can be carried on the partitions of the honeycombstructure. The honeycomb structure may be made of ceramics.Alternatively, a metallic matrix having a net structure may have asurface coated with TiO₂. Or, a fibrous matrix made of glass may have asurface coated with TiO₂. Alternatively, a photocatalyst can be carriedon a surface of a light source so as to integrate the photocatalyst withthe light source as stated hereinafter (Japanese Patent Application No.Hei-8-31231, the disclosure of which is incorporated herein byreference).

A photocatalyst can be carried on a matrix by any of well knownprocesses including a sol-gel process, sintering process, evaporationprocess, sputtering process, coating process, baking finish process andthe like. Materials and shapes of these matrices as well as the mannerof carrying catalytically active components can be selected asappropriate, depending on the size and shape of an apparatus, types andshapes of a light source, kinds of catalytically active-components,desired effects, economical efficiency and the like. A method forsupporting catalytically active components on a linear article such as afiber according to sol-gel process is described in the Laid OpenJapanese Patent Application No. Hei-7-256089, the disclosure of which isherein incorporated by reference.

A photocatalyst may be disposed in a space where a gas to be treatedflows through. Catalytically active components may be coated on thesurface of the walls, floors and ceilings that define a space prevailedby the flow of a gas to be treated.

Any light source may be used for light irradiation, provided that it canemit light having a wavelength absorbable with a photocatalyst. Lightbelonging to the visible and/or ultraviolet regions is effective, andhence an U lamp or the sunlight can be appropriately applied. Exampleswhich can be mentioned are a bactericidal lamp, a black light, afluorescent chemical lamp, an UV-B lamp and a xenon lamp. Radiations asmentioned hereinbefore can be appropriately used. Materials ofphotocatalysts, materials and shapes of matrices, the presence or not ofadditives, types of irradiation sources and their installation mode in agas to be treated can be appropriately selected on the basis of theresults of preliminary experiments, depending on fields of applicationas well as sizes, shapes and required specifications of an apparatus.

The mechanisms by which organic compounds can be transformed intoparticles and the resultant particles of particulate matter can bedecomposed in the presence of a photocatalyst are generally believed tobe as described below, though many details are remaining unclear due tothe fact that organic compounds are thought to be present in the air inthe form of a mixture of more than hundreds or thousands ofconstituents.

Organic compounds in the air can be activated themselves upon exposureto an ultraviolet ray and/or a radiation ray. When water is present inthe air even at a slight concentration, the water may induce theproduction of OH radical and/or provoke an ionic nucleation reaction.Reaction products resulted from these many complicated reactions areassociated to become microparticles. Even when the organic compounds arepresent in an extremely low concentration, reactions can occureffectively. For example, phthalic esters such as siloxane, DOP and DBPcan be easily transformed into microparticles.

The surface of a photocatalyst is activated upon irradiation of lightand/or radiation. Contaminants that can easily deposit on a substratesuch as a wafer can easily deposit also on a photocatalyst also. Organiccompounds such as phthalic esters are hydrophilic, and therefor caneasily deposit onto an active surface. Moreover, those organic compoundsare concentrated in the form of microparticles, and therefore moreeasily deposit onto the surface of a photocatalyst. Subsequently, thecontaminants are decomposed on the surface of the photocatalyst into astable form of a low molecular weight.

Purified gas in a clean room contains organic compounds that have beendecomposed and hence cannot deposit on a wafer and a glass substrate.Even though a component in the purified gas deposits on a wafer, thecomponent is hydrophilic. Therefore in either case, the contact angledoes not increase. In other words, exposing a wafer and a glasssubstrate to the purified gas according to the present invention, doesnot increase the contact angle.

Since organic compounds comprise many constituents as mentioned above,it is practically not possible to analyze and estimate completely thecomposition thereof. Contamination of a substrate with organic compoundsdepends on the activity of each surface (e.g. film-forming species). Inother words, contamination is different depending on the surface stateof the substrate. A sensitive substrate is largely influenced bycontaminants. According to the present invention, by taking non-methaneorganic compounds as an indicator, the non-methane organic compounds maybe removed to a level preferably of not more than 0.2 ppm, morepreferably of not more than 0.1 ppm.

Non-methane organic compounds are taken as an indicator because they canbe easily measured by gas chromatography (GC). In contrast theconstituents that can easily deposit on a substrate and consequentlybecome troublesome in a clean room, such as siloxane and DOP(practically problematic matter) are present in an extremely lowconcentration of at most 1 ppb, and therefore the measurement andanalysis thereof are complicated and laborious as well as are difficultto be monitored.

According to the present invention, when oxygen is present in a gas,ozone is generated upon irradiation of an ultraviolet ray and/or aradiation ray. The resultant ozone is preferably removed by any of wellknown ozone-decomposing materials, depending on fields of application.In the fabrication of a silicon substrate for example, the surface ofthe silicon substrate may be oxidized into silicon dioxide in thepresence of ozone.

Ozone-decomposing materials which can be used are composite oxidecatalysts including manganese dioxide catalysts, MnO₂/TiO₂—C,MnO₂/ZrO—C, as previously proposed by the present inventors (Laid OpenJapanese Patent Application No. Hei-6-190236). Well-known activecharcoals can be suitably used as well. Ozone-decomposing materials aredescribed in the Laid Open Japanese Patent Application No. Hei-6-190236,the disclosure of which is incorporated herein by reference.

Ozone can be decomposed also in the presence of a photocatalyst used inthe present invention, but it is preferred to use any ofozone-decomposing materials set forth above, in the case when theresultant ozone reaches a high concentration or when a tolerable ozoneleakage is at only a low level.

Preferably, the method of the present invention comprises further a stepof removing microparticles of the contaminant from a gas. Preferably,the apparatus of the present invention has a section for removingmicroparticles of the contaminant. More precisely, it is preferable totrap and remove microparticles of the contaminant with a filter or anadsorbent or by charging with photoelectrons. This removal step caneliminate mainly gaseous contaminants other than organic compounds, suchas sulfur oxides (SOx), nitrogen oxides (NOx) and ammonia (NH₃) in theform of microparticles.

It is preferred according to the present invention that acidic and basicgases as well as microparticles are removed. Removal can be carried outby using one or more of a filter, an adsorbent and an ion exchangerand/or by electrically charging the contaminant with photoelectronsfollowed by trapping the charged contaminant. Organic compounds andorganosilicon compounds that had been transformed into microparticlescan also be removed by these removing means.

Filters which can be used include a HEPA filter, an ULPA (Ultra LowPenetration Air) filter, an electrostatic filter, an electret material,an ion exchange filter and the like. The ion exchange filter ispreferred depending on fields of application, since it can trap a toxicgas, an odorous gas and the like which partly flows out withouttransformed into microparticles if present.

Adsorbents which can be used include active charcoal, silica gel,synthetic zeolite, molecular sieve and alumina. Alternatively, theadsorbents composed of a glass and a fluororesin that were previouslyproposed by the present inventors for trapping non-methane organiccompounds can be suitably used (Laid Open Japanese Patent ApplicationNo. Hei-6-324). The disclosure about adsorbents described in the LaidOpen Japanese Patent Application No. Hei-6-324 is herein incorporated byreference. According to the present invention, gaseous contaminantsdifficult to remove as such can be effectively removed by theseadsorbents, since the contaminants are transformed into particles bymeans of an ultraviolet ray and/or a radiation ray.

In UV/photoelectron systems, microparticles are electrically chargedwith photoelectrons emitted from a photoelectron-emitting material andthe resultant charged microparticles are trapped and removed. TheU/photoelectron systems previously proposed by the present inventors canbe applied as desired. Methods for removing contaminants in the form ofcharged microparticles are described in the Japanese Patent PublicationsNos. Hei-3-5859, Hei-6-34941, Hei-6-74909, Hei-6-74910, Hei-8-211,Hei-7-121369 and Hei-8-22398, all of which are herein incorporated byreference.

Any photoelectron-emitting material can be used, provided that it canemit photoelectrons upon UV irradiation. Those materials having a lowervalue of photoelectrically working function are preferred. In view ofeffects and economical efficiency, any of Ba, Sr, Ca, Y, Gd, La, Ce, Nd,Th, Pr, Be, Zr, Fe, Ni, Zn, Cu, Ag, Pt, Cd, Pb, Al, C, Mg, Au, In, Bi,Nb, Si, Ti, Ta, U, B, Bu, Sn and P as well as compounds, alloys andmixtures thereof can be preferably used alone or in combination of twoor more of them. Physical composite materials such as amalgam can beused as well.

Suitable compounds include oxides, borides and carbides. Examples ofoxides are BaO/SrO, CaO, Y₂O₅, Gd₂O₃, Nd₂O₃, ThO₂, ZrO₂, Fe₂O₃, ZnO,CuO, Ag₂O, La₂O₃, PtO, PbO, Al₂O₃, MgO, In₂O₃, BiO, NbO, BeO and thelike. Examples of borides are YB₆, GdB₆, LaB₆, NdB₆, CeB₆, BuB₆, PrB₆,PrB₆, ZrB₂ and the like. Examples of carbides are UC, ZrC, TaC, TiC,NbC, WC and the like.

Alloys which can be used are brass, bronze, phosphor bronze, an alloy ofAg with Mg (2 to 20 wt % of Mg), an alloy of Cu with Be (1 to 10 wt % ofBe) and an alloy of Ba with Al. Ag—Mg alloy, Cu—Be alloy and Ba—Al alloyas mentioned above are preferred. Oxides can be obtained also by heatingonly the surface of a metal in the air or by oxidizing it with achemical agent.

Alternatively, an oxide layer having a good long-term stability may beformed on the surface of a metal alloy by heating the metal alloy priorto use. By way of an example, a surface oxide layer can be formed byheating a Mg—Ag alloy in steam at a temperature of 300° C. to 400° C.The obtained oxide thin layer exhibits a good long-term stability.

Photoelectron-emitting materials having a multi-layer construction aspreviously proposed by the present inventors can also be used. Thedisclosure about photoelectron-emitting materials described in the LaidOpen Japanese Patent Application No. Hei-1-155857 is herein incorporatedby reference. Alternatively, a photoelectron-emitting substance can befabricated by forming a thin layer carried on a suitable matrix. Forexample, a thin layer of Au carried on a glass matrix may be used.

These materials can be used in any shape of a plate, a pleat, a curvedsurface, a net and the like. Shapes having a larger UV-exposed area anda larger air-contacting area are preferred.

Photoelectrons can be effectively emitted from a photoelectron-emittingmaterial by suitably applying a reflective surface or a curvedreflective surface, as previously proposed by the present inventors. Thedisclosure about photoelectron-emitting materials described in theJapanese Patent Publication No. Hei-6-34941 is herein incorporated byreference. Alternatively, an integral photoelectron-emitting device maybe formed by coating an U lamp with a photoelectron-emitting material asstated below. The disclosure about photoelectron-emitting devicesdescribed in the Laid Open Japanese Patent Application No. Hei-4-24354is herein incorporated by reference. Shapes of photoelectron-emittingmaterials and those of reflective surfaces may be varied depending onshapes and constructions of a device or desired efficiency and can bedetermined as appropriate.

Any ultraviolet radiation may be used, provided that it can irradiate aphotoelectron-emitting material so as to emit photoelectrons. A mercurylamp, a hydrogen discharge tube, a xenon discharge tube, a Lymandischarge tube, etc. are generally suitable. Those having a bactericidal(sterilizing) action at the same time are preferred depending on fieldsof application. Types of ultraviolet rays can be selected as appropriatedepending on fields of application, operation details, purposes of use,economic efficiency and the like. In the biological field for example,the combined use of far infrared rays is preferred in view ofbactericidal action and efficiency. A bactericidal lamp (main wavelengthof 254 nm) is preferable, because the electrically charging action ofthe present invention is added with a bactericidal action. Any UV sourcecan be used, provided that it can emit an ultraviolet radiation. The UVsource can be selected as appropriate depending on fields ofapplication, shapes and sizes of a device, effects and economicalefficiency.

By irradiating the photoelectron-emitting material with an ultravioletradiation in an electrical field, photoelectrons can efficiently supplymicroparticles with an electrical charge. Disclosures about electricalcharging in an electrical field described in the Laid Open JapanesePatent Applications Nos. Sho-61-178050 and Sho-62-244459 as well as LaidOpen Japanese Patent Application No. Hei-1-120563 are hereinincorporated by reference. In the present invention, an electrical fieldmay range from 0.1 V/cm to 5 kV/cm and the suitable intensity thereofcan be determined as appropriate based on the results of preliminaryexperiments and examinations.

Since photoelectrons can electrically charge even extremely minutemicroparticles (e.g. those having a particle size of <0.1 μm) with ahigh efficiency, microparticles can be trapped and removed efficiently.Prior to electrical charging, microparticles may be made to grow to alarger particle diameter. A method for growing and chargingmicroparticles electrically has been previously proposed by the presentinventors (the Japanese Patent Application No. Hei-1-120564) and can besuitably used for electrically charging extremely minute microparticlesas desired, depending on fields of application.

Any trapping material can be used, provided that it can trap theelectrically charged microparticles. Dust-collecting plates(dust-collecting electrodes) and electrostatic filter systems are ofgeneral use, but an effective trapping material can also be constructedin such a way that the trapping section itself forms an electrode madeof a woolen material such as steel wool or tungsten wool (woolenelectrode material). Electret materials can be suitably used, too. Anion exchange filter (fiber) that has been previously proposed by thepresent inventors may be effective depending on fields of application.Ion exchange filters can trap coexistent gaseous contaminants andodorous gases that are difficult to be trapped by the present inventionand hence the use of ion exchange filters is preferred depending onfields of application. These trapping materials may be used alone or incombination of two or more of them as appropriate, depending on fieldsof application, sizes and shapes of a device, economic efficiency andthe like.

An ion exchange material refers to a support having the surface attachedby an ion exchanger or an ion exchange group. Ion exchangers includecationic ion exchangers and anionic ion exchangers, a combination ofboth being preferred. Ion exchange groups include cationic ion exchangegroups and anionic ion exchange groups, a combination of both beingpreferred.

Ion exchange materials are preferably ion exchange fibers having asupport composed of a fiber. Fibers which can be used are naturalfibers, synthetic fibers and the mixtures thereof.

Ion exchangers are now be described mainly with reference to ionexchange fibers. Ion exchangers may be supported on a fibrous supportdirectly or on a support composed of a woven, knitted or filled fiber.Any form of fibers can be used, provided that ion exchangers supportedon a fiber can be finally obtained.

Preferable methods for fabricating an ion exchange fiber suitable foruse in the present invention are those that imply a graftpolymerization, in particular radiation graft polymerization. This isbecause these methods can make use of starting materials having variousproperties and sizes.

Natural fibers which can be used include wool, silk etc. and syntheticfibers which can be used include hydrocarbon polymer-based fibersfluorine-containing polymer-based fibers or polyvinyl alcohol,polyamide, polyester, polyacrilonitrile, cellulose, cellulose acetate,etc.

Hydrocarbon polymers which can be used include: aliphatic polymers suchas polyethylene, polypropylene, polybutylene and polybutene; aromaticpolymers such as polystyrene and poly-α-methylstyrene; cycloaliphaticpolymers such as polyvinyl cyclohexane; or copolymers thereof.Fluorine-containing polymers which can be used include polyethylenetetrafluoride, polyvinylidene fluoride, ethylene-ethylene tetrafluoridecopolymer, ethylene tetrafluoride-propylene hexafluoride copolymer,vinylidene fluoride-propylene hexafluoride copolymer and the like.

Any material can be used as a support, provided that it: has a largecontact area with a gas stream; is shaped to have a diminishedresistance; is readily to be grafted; has a good mechanical strength; isless likely to produce and fall waste fiber; and is less influenced byheat. A suitable support material can be selected depending on purposesof use, economical efficiency and effects, but generally a support ismade of polyethylene, and most preferably made of polyethylene or acomposite of polyethylene and polypropylene.

Various cationic ion exchangers and anionic ion exchangers can be usedas an ion exchanger without particularly being limited. Examples whichcan be mentioned are cationic ion exchangers that contain a cationic ionexchange group such as a carboxyl, sulfonate, phosphate or phenolicgroup as well as anionic ion exchangers that contain a cationic ionexchange group such as any of primary to tertiary amino groups or aquaternary ammonium group or ion exchangers having both of the aforesaidcationic and anionic ion exchange groups.

More precisely, fibrous ion exchangers having a cationic or anionic ionexchange group can be obtained by graft polymerizing on the aforesaidfiber a styrene compound such as acrylic acid, methacryric acid,vinylbenzene-sulfonic acid, styrene, halomethylstyrene, acyloxystyrene,hydroxystyrene or aminostyrene, or, vinyl pyridine,2-methyl-5-vinylpyridine, 2-methyl-5-vinylimidazole or acryronitrilefollowed by reacting with sulfuric acid, chlorsulfonic acid or sulfonicacid as required. Optionally, these monomers may be grafted on a fiberin the presence of a monomer having two or more double bonds such asdivinyl benzene, trivinyl benzene, butadiene, ethylene glycol, divinylether, ethylene glycol dimethacrylate and the like.

Ion exchange fibers can be fabricated in the manner as described above.Ion exchange fibers have a diameter of 1 to 1,000 μm, preferably 5 to200 μm and the diameter can be appropriately selected depending on typesand uses of a fiber.

The way of using cationic ion exchange groups and anionic ion exchangegroups in these ion exchange fibers can be determined depending on thekinds and the concentrations of the components to be removed in a gas tobe purified. By analyzing the gas previously and estimating thecomponents to be removed, adequate types and amounts of ion exchangefibers can be selected. More precisely, when an alkaline gas is to beremoved, fibers having cationic ion exchange groups (cation exchangers)are suitable, whereas, when an acidic gas is to be removed, fibershaving anionic ion exchange groups (anion exchangers) are suitable. And,when both an alkaline and acidic gases are to be removed, both ofanionic ion exchange groups and cationic ion exchange groups may beused.

An effective way of flowing a gas through an ion exchange fiber is togenerate a gas stream perpendicular to a filter made of the ion exchangefiber.

The flow rate of a gas passing through an ion exchange fiber can besuitably determined by conducting preliminary experiments. Since havinga high removal rate, this fiber can be used generally at a SV of theorder of 1,000 to 100,000 (h⁻¹). Ion exchange fibers that are producedby a radiation graft polymerization, as previously proposed by thepresent inventors, can be suitably used with a particularly highefficiency (Japanese Patent Publications Nos. Hei-5-9123, Hei-5-67325,Hei-5-43422 and Hei-6-24626).

Ion exchange fibers are effective in trapping of ionic matter(constituents) and hence can efficiently trap and remove the ionicmatter intended by the present invention.

In particular, ion exchange filters (fibers) that are fabricated byradiation graft polymerization are practicably effective, since theradiation can reach deep in a support uniformly and the ion exchanger(anion and/or cationic exchanger) can firmly attach over a large areathereof (at a high density) so that the exchange volume is made largerand hence ionic matter in a low concentration can be removed at a highrate and with high efficiency.

Fabrication process using a radiation graft polymerization isadvantageous in that: the polymerization can be conducted on a supporthaving the shape near to that of finished product; it can be conductedat room temperature; it can be conducted in vapor phase; it can realizea larger grafting ratio; and it can give an adsorption filter having alow impurity content.

As the result, following characteristics can be obtained:

(1) Ion exchange fibers made by radiation graft polymerization canexhibit a higher adsorption rate and a larger adsorption amount, sincean ion exchanger (adsorptive part) is added thereto more uniformly andmore abundantly (a higher addition density).

(2) Pressure loss can be diminished.

A means for trapping and removing microparticles may be used alone or incombination of two or more, which can be suitably selected on the basisof preliminary experiments, depending on the properties of the producedmicroparticles.

Schemes for producing microparticles (irradiation source, way ofproducing microparticles, conditions and the like) and for trapping andremoving microparticles (trapping means, conditions and the like) can besuitably selected on the basis of preliminary experiments, depending onvarious factors including application field, gas type, apparatus design,production scale, performance requirement, economical efficiency and thelike.

The method of the present invention for removing microparticles ofcontaminants can be carried out according to any of the 6 schemes shownbelow by way of examples.A→B→C  (1)A→B→A→C  (2)A→C→A→B  (3)A→C→B  (4)A→B+C  (5)A+B→C  (6)

In the schemes above:

the member A represents a step for irradiating a gas with an ultravioletray and/or a radiation ray; or a microparticle-producing section havinga source of an ultraviolet ray and/or a radiation ray;

the member B represents a step for contacting the microparticles ofcontaminants with a photocatalyst and for irradiating the photocatalystbeing kept in contact with the microparticles; or a decompositionsection having a photocatalyst and a light source for irradiating thephotocatalyst;

the member C represents a step for clearing the gas of themicroparticles of contaminants; or a removal section for removing themicroparticles of contaminants;

the member C isn't essential to the present invention; the symbol →indicates a temporal sequence of the steps; or a spatial sequence of thesections downstream; and

the symbol + indicates concurrence of 2 steps; or integration of 2sections.

Suitable schemes can be selected on the basis of preliminary experimentsand examinations, depending on fields of application, types ofapparatus, states of the gas to be treated, required removalperformances, economical efficiency and the like.

Generally speaking,

(a) when an apparatus is of a large size, the schemes are preferred inthe order of (2), (3)>(1), (4)>(5),

(b) when organic compounds are present in a concentration higher thanthat of other gaseous contaminants, (1) or (2) is preferred,

(c) when a gas contains a high concentration of constituents having apossibility of becoming a catalytic poison to photocatalysts during along-term operation, such as sulfur-containing compounds, (3) or (4) ispreferred,

(d) when a high removal performance is required, (2) or (3) ispreferred: and

(e) when an apparatus is of a small size, (5) or (6) is preferred.

The member C may precede the members A and B. This order is preferredbecause acidic gases having a possibility of adversely influencing onphotocatalysts can be preliminarily eliminated. Such acidic gasesinclude SO₂, NO, HCl and HF. Moreover, when sulfur-containing compounds,such as sulfur oxides, hydrogen sulfide, thiophene and thiols arepresent in a high concentration, the compounds might act onphotocatalysts as catalytic poison.

EXAMPLES

The present invention will now be described in more detail by way ofexamples, which should not be construed as limiting the scope of thepresent invention

Example 1

FIG. 1 represents an embodiment of the apparatus of the presentinvention applied to purify air for feeding an air knife in asemiconductor production factory. A purification apparatus 12 of thepresent invention is placed in a clean room 1 of class 1,000.

First, a device for feeding air 14 into the clean room 1 is described.An air inlet pipe 2 is connected to a pre-filter 3 for filtering offcoarse solid particles present in the outside air. The pre-filter 3 andan air outlet 4 of the clean room 1 are connected to a fan 5 for feedingair into the clean room 1. The fan 5 is connected to an air-conditioner6 for controlling the temperature and the humidity of the air. The pipeleaving the air-conditioner is divided into several branches, eachbranch being connected to a HEPA filter 7 for removing solidmicroparticles. The HEPA filter 7 is provided at the interface with theinternal space of the clean room 1, and preferably at the ceiling of theclean room. The air outlet 4 is provided at the bottom of the clean room1.

The outside air 2 to be fed into the clean room 1 is treated first withthe coarse filter 3 and the air-conditioner 6. The air is then freedfrom dust with the HEPA filter 7 before entering into the clean room 1,to become air 14 of class 1,000 containing an extremely lowconcentration of organic compounds 30. In other words, the organiccompounds of an extremely low concentration originating in organicmaterials (polymer materials) including cars and plastics can be removedwith neither coarse filter 3, nor air-conditioner 6, nor HEPA filter 7 ,and hence are brought to enter into the clean room 1. On the other hand,the structural components of the clean room 1 are evolving organiccompounds in the clean room 1. In consequence, the air in the clean room1 has a concentration of organic compounds higher than that of theoutside air, The air 14 in the clean room 1 has a concentration oforganic compounds of 0.8 to 1.2 ppm in terms of non-methane hydrocarbonstaken as an example.

Preferably, the purification apparatus 12 of the present invention ishorizontally disposed. As the result, the air 14 can flow horizontallythrough the apparatus 12. Preferably, an air knife device 29 is providedwith an air inlet in the direction of exhaust air discharged from an airoutlet of the purification apparatus 12. And preferably, the air outlet4 of the clean room 1 is provided in the direction of exhaust airdischarged from the air outlet of the air knife 29.

As shown in FIG. 1, the air 14 is fed through the HEPA filter 7 into theclean room 1 downwardly from the top thereof. Then, while passingthrough the purification apparatus 12 horizontally the air 14 is freedfrom dust and is purified to become clean air 28 in which organiccompounds had been decomposed. The resultant clean air 28 is fed intothe air knife 29 for cleaning a wafer. Exhaust air of the air knife 29is drawn out of the air outlet 4 provided at the lower part of the cleanroom.

As shown in FIG. 1 the purification apparatus 12 is providedsuccessively with a coarse filter 25 a for dust-removing, amicroparticle-producing section 8, a decomposition section 26, a coarsefilter 25 b and an ozone-decomposing section 27.

Preferably, the purification apparatus 12 has the coarse filter 25 a.Dust that is eventually present in the clean room 1 tends to contaminateand deteriorate a photocatalyst 32. The coarse filter 25 a are usefulfor removing such dust.

The microparticle-producing section 8 consists of a housing and an UVlamp 15 received therein. The U lamp is a low-pressure mercury lamp 15,for example.

Organic compounds 30 in the air 14 have a possibility of increasing thecontact angle by depositing onto a substrate such as a wafer. But, whenirradiating the air 14 with the UV radiation emitted by the UV lamp 15,these organic compounds 30 can be transformed into particulate matter16. Upon exposure to the UV radiation, gaseous oxygen present in the airalso is transformed into gaseous ozone.

The decomposition section 26 consists of an UV lamp 31 and a pair ofphotocatalysts 32 placed opposite each other on the either side of the Ulamp in the air-flowing direction. The photocatalyst 32 shown in FIG. 1is composed of a honeycomb structure provided with the partitionsdefining two or more through-holes and a catalytically active componentin the form of particles. The catalytically active component is made oftitanium dioxide in the form of particles that are coated on the surfaceof the partitions of the honeycomb structure. Preferably, the honeycombstructure has the length in the direction parallel to the partitionsshorter than that in the radial direction. This is to avoid theinterception of UV radiation by the partitions.

The air which contains organic compounds passes through thethrough-holes of the honeycomb structure and are introduced into thedecomposition section 26. While the air 14 is passing through the holesof the honeycomb structure of the photocatalyst 32, particulate matter16 contained in the air 14 is brought into contact with thecatalytically active components in the form of particles present on thesurface of the through-holes of the honeycomb structure. Since thephotocatalyst 32 is activated by irradiation with the UV lamp 31,particulate matter is decomposed under catalytic action. Ultimately,organic compounds are reduced to a level of not more than 0.2 ppm,preferably 0.1 ppm in terms of non-methane organic compounds taken as anindicator. Organic compounds having a high molecular weight or activeorganic compounds that are likely to increase the contact angle on awafer are decomposed into organic compounds of a low molecular weightthat aren't likely to increase the contact angle, or carbon dioxide andwater.

The dust-removing filter 25 b consists of an ULPA filter, for example.The air of class 1,000 in the clean room 1 can be cleared ofmicroparticles with the ULPA filter to become class 10 or less. Thedust-removing filter 25 b can also efficiently trap microparticlesescaped at or near the microparticle-producing section 8 in case ofemergency. Preferably, the ozone-decomposing material 27 has a honeycombshape provided with the partitions defining at least 2 through-holes.Ozone that was generated upon irradiation with the UV lamp 15 can bedecomposed to a level of not more than 0.01 ppm by means of thephotocatalyst 32 and the ozone-decomposing material 27. Since generationof ozone cannot be ignored in semiconductor production factories, ozoneis decomposed to a level equal to or less than that in natural airthrough the two-step decomposition.

Example 2

Example 2 is described with reference to FIG. 2.

As shown in FIG. 2, a wafer stocker 36 for storing wafers is disposed inthe clean room 1 of class 1,000 and a purification apparatus 12 isdisposed in the stocker 36. The purification apparatus 12 comprises amicroparticle-producing section 8 for transforming organic compoundsinto microparticles by UV irradiation, a decomposition section 26 fordecomposing the resultant microparticles of organic compounds by theaction of a photocatalyst and an ozone-decomposing section 27. The air14 in the stocker 36 containing organic compounds 30 is treated with thepurification apparatus 12 of the present invention to become clean air28. Organic compounds in this air 28 are decomposed to a level of notmore than 0.01 ppm, at which level the organic compounds can avoidincreasing the contact angle on the surface of a wafer.

This example will be described in more detail below.

The air 14 in the stocker 36 is first irradiated with an UV lamp 15(low-pressure mercury lamp) so that organic compounds 30 containedtherein are transformed into particulate matter 16. This particulatematter 16 are made to deposit onto the surfaces of the photocatalyst 32(carried on and fixed to the surrounding wall surfaces and the glassrods) which had been irradiated with an UV lamp 31 (bactericidal lamp).The deposited particulate matter is decomposed and removed throughphotocatalysis to afford a clean air 28.

As shown in FIG. 2, the photocatalyst 32 is composed of at least 2 glassrods and titanium oxide in the form of particles applied onto thesurface of each glass rod. This photocatalyst is hanging. Thephotocatalyst is further applied on the wall surfaces. Moreparticularly, titanium oxide in the form of particles dispersed in asuitable solvent are applied on the wall surfaces.

By opening the wafer stocker 36, the air of class 1,000 in the cleanroom enters into the wafer stocker 36. This air contains organiccompounds in a concentration of 0.8 to 1.5 ppm in terms of non-methaneorganic compounds. The air in the form of air streams 33 a, 33 b and 33c containing these organic compounds are made to contact with thephotocatalyst 32, and consequently microparticles of organic compoundshaving a high molecular weight or those of active organic compounds canbe effectively decomposed into carbon dioxide and water. Organiccompounds are decomposed to a level below 0.1 ppm in terms ofnon-methane organic compounds taken as an indicator.

Irradiation with the UV lamps 15, 32 makes the temperature elevate andgives rise to air convection. As a result of this air convection, airflows upwardly from the bottom towards the top of the apparatus 12, andoutside of the apparatus, air streams 33 a, 33 b and 33 c are generated.

Since the air stream inside the apparatus 12 gives rise to Brownianmovement on a molecular level, contaminants are brought into collisionwith the glass rods or the wall surfaces and hence microparticles oforganic compounds are made into contact with the photocatalyst. Sincecontaminants are easy to deposit onto a wafer, contaminants can depositalso onto the photocatalyst.

In the manner as described above, the part for storing the wafers 34 setin a wafer case 35 is effectively cleaned.

Ozone generated upon irradiation with the UV lamp 15 for producingparticles can be decomposed and removed to a level of not more than 0.01ppm by the action of the photocatalyst 32 and an ozone-decomposingmaterial 27 of a honeycomb shape.

In FIG. 2, reference numerals identical to those in FIG. 1 denote thesame elements as those in FIG. 1.

Though the cases when a gas is composed of air are described in theexamples 1 and 2, the present invention can of course be applied as wellto the cases in which gas is another gas such as nitrogen or argon thatcontains gaseous contaminants such as organic compounds as impurities.The present invention can be applied not only under atmosphericpressure, but also under an increased or decreased pressure.

Example 3

FIG. 3 shows a purification apparatus 12 of a type different from thatshown in FIG. 2. As shown in FIG. 3, the order of the ozone-decomposingmaterial 27 and the decomposition section 26 provided with thephotocatalyst are reversed so that the ozone-decomposing material 27 issituated next to the particle-producing section 8. The apparatus thusarranged has the effects comparable to those of the apparatus shown inFIG. 2.

Example 4

A sample of gas as specified below was charged into a stocker arrangedas shown in FIG. 2, a wafer was placed therein, and then the contactangle on the wafer, the concentrations of non-methane organic compoundsand ozone present in the stocker were measured.

Experimental Conditions:

(1) sample gas: air of class 10 in a semiconductor production factorycontaining non-methane organic compound in a concentration of 0.8 to 1.2ppm;

(2) stocker volume: 30 liters;

(3) UV lamp for producing particles: low pressure mercury lamp (184 nm);

(4) photocatalyst: titanium dioxide carried on a glass fiber matrix bysol-gel process;

(5) light source: bactericidal lamp (254 nm) (for irradiating aphotocatalyst);

(6) ozone-decomposing material: composite oxide catalyst, MnO₂/ZrO—C.

More precisely, the ozone-decomposing material is composed of ahoneycomb structure provided with the partitions defining at least 2through-holes and a manganese oxide coated on the surfaces of thepartitions of the honeycomb structure. The honeycomb structure issubstantially composed of a zirconium oxide and carbon atoms. At least aportion of carbon atoms may be present in the form of zirconium carbide.Manganese oxide has the shape of a particle, for example. Theseparticles are not always required to cover all the surfaces of thepartitions of the honeycomb structure.

(7) wafer: a highly pure silicon wafer of 5-inch diameter was cut intopieces of 1 cm×8 cm and placed in the stocker;

(8) pretreatment of the wafer: washing with a detergent and alcohol on aclean bench in the clean room, followed by UV/O₃ cleaning;

(9) measurement of the contact angle: the contact angle was measuredwith a CA-D type contact angle feeler manufactured by Kyowa KaimenKagaku, Inc.;

(10) concentration of non-methane organic compounds: was measured by gaschromatography (GC) method;

(11) ozone concentration: was measured with a chemiluminescent ozonedensitometer;

(12) opening of the stocker: the stocker was disposed in a clean zone(class 10) of a semiconductor production factory and open-close cycleswere repeated 6 times per day.

Results:

(1) FIG. 4 shows the contact angle on a wafer as the function of thenumber of days during which the wafer was stored in a stocker.

In FIG. 4, the symbol ◯ represents the values obtained according to thepresent invention and the symbol ● represents the values obtained from a(comparative) test in which the wafer was exposed to the air of class 10in a clean room. The symbol ↓ means that the obtained value is below thelimit of detection.

(2) Concentrations of non-methane organic compounds and ozone present inthe stocker are shown in the Table 1. TABLE 1 storing period organiccompound ozone (days) concentration(ppm) concentration 1 <0.1 ppm <0.01ppm 2 <0.1 ppm <0.01 ppm 10 <0.1 ppm <0.01 ppm

Switching the UV lamp for producing particles generates a ozoneconcentration of 15 to 20 ppm.

(3) In a stocker without the unit of the present invention, a wafer wasstored for 2 and 7 days. Then, the wafer was taken out of the stocker,heated to desorb the organic compounds deposited thereon. Analysis ofthe wafer by gas chromatography/mass spectrometry (GC/MS) showed thepresence of phthalic esters such as DOP. The unit of the presentinvention was disposed in the stocker and a wafer was treated andanalyzed in the same way. Phthalic esters such as DOP was undetectable.

Example 5

FIG. 5 shows an embodiment of the apparatus of the present inventionapplied in a semiconductor production factory to purify air for feedingan air knife. The purification apparatus 12 shown in FIG. 5 is differentfrom that shown in FIG. 1 in having further a section C for removingparticulate matter.

As shown in FIG. 5, reference numeral 1 represents a clean room of class100. The air 14 in the clean room 1 is treated with the purificationapparatus 12 of the present invention comprises a section 8(A) forproducing microparticles of organic compounds and coexistent gaseouscontaminants such as SO₂ by UV irradiation, a section 26(B) fordecomposing the microparticles of organic compounds with aphotocatalyst, an ozone-decomposing section 27 and the sections 9, 10(C)for trapping the microparticles of particulate matter withphotoelectrons. The air 14 having passed through the purificationapparatus 12 of the present invention results in the clean air 28 whichis free from dust as well as organic compounds and coexistent gaseouscontaminants. The clean air 28 is fed to an air knife device 29 forcleaning a wafer (substrate).

This embodiment is, as described above, arranged according to thefollowing scheme. section for producing microparticles by UW irradiation(A) →decomposition section for decomposing microparticles of organiccompounds with a photocatalyst (B)→removal (dust-removal) section fortrapping and removing particulate matter in the form of microparticles(C).

This embodiment is described hereinafter in more detail.

The outside air 2 is treated first with a coarse filter 3 and anair-conditioner 6 before entering into the clean room 1. Then, the airis cleared of dust with a HEPA filter 7 at the inlet of the clean roomto become air 14 having a microparticle concentration of class 100. Inthe air 14, there exist organic compounds 30 a together with gaseouscontaminants 30 b which are composed of acidic gases including SOx, NOx,HCl, HF, etc. and basic gases including NH₃, amines, etc. Acidic andbasic gases contained in the outside air 2 are carried into the cleanroom 1 in company with the outside air 2. Only a little amount of acidicand basic gases generate in the clean room 1. The air 14 in the cleanroom has an organic compound concentration of 0.8 to 1.2 ppm in terms ofnon-methane organic compounds.

In the air 14 in the clean room 1, organic compounds 30 a as well asacidic and basic gases 30 b are transformed into particulate matter 16upon irradiation of the UV lamp (low-pressure mercury lamp) 15(A).

Among the particulate matter 16, particles that are derived from organiccompounds 30 a and basic gas tend to deposit onto the surface of thephotocatalyst (TiO₂), and hence can deposit on the surface of thephotocatalyst 32 activated by irradiation with the UV lamp 31 so as tobe decomposed and removed through photocatalysis (B).

On the other hand, among the particulate matter 16, particles derivedfrom acidic gas are difficult to decompose with the photocatalyst 32,and consequently pass through the photocatalytically decomposing sectionB into the section (C) for trapping and removing with the aid ofphotoelectrons. The section (C) is composed of an UV lamp 19, aphotoelectron emitting material 20, an electrode (part 9 for supplyingparticulate matter with an electrical charge) and a material 10 fortrapping the electrically charged particulate matters 10.

In the trapping and removing section (C), influent particulate matter 16are electrically charged with photoelectrons (not shown) that areemitted from the photoelectron emitting material 20, and theelectrically charged particulate matter is trapped and removed with thematerial 10. Photoelectrons can be efficiently emitted by covering theUV lamp 19 with the photoelectron emitting material 20 and forming anelectrical field of 50 V/cm between the material 20 and the electrode21.

In this embodiment, air contaminated with organic compounds isintroduced into the decomposing section 26 (B) which is composed of aphotocatalyst 32 (formed by coating titanium oxide on the surface of thepartitions of a honeycomb-shaped ceramic matrix) and an UV lamp(bactericidal lamp) 32. In this section, organic compounds aredecomposed to a level of not more than 0.2 ppm, preferably 0.1 ppm interms of non-methane organic compounds taken as an indicator.

More precisely, organic compounds of a high molecular weight or activeorganic compounds that are responsible for an increase in the contactangle are decomposed into organic compounds of a low molecular weightthat avoid any increase in the contact angle, or into carbon dioxide andwater.

The resultant air has a concentration of acidic and basic gases by anamount tenfold lower than that of the air 14 in the clean room. In termsof SO₂ taken as an indicator, an average SO₂ concentration which hadbeen 0.001 ppm (10 ppb) in the clean room 1 can be decreased to 1 ppb orless.

In a manner as described above, microparticles present in the air 14 ofclass 100 in the clean room are, together with particulate matter 16originating from gaseous contaminants, are electrically charged andtrapped at the removal section (C) for removing contaminants withphotoelectrons. By virtue of the apparatus of the present invention 12,the air 28 becomes an extremely clean air which is superior to air ofclass 1 and are free from organic compounds and coexistent gaseouscontaminants.

Ozone that is generated by irradiation of the UV lamp 15 for producingparticles can be decomposed to a level of not more than 0.01 ppm bymeans of the photocatalyst 32 and a honeycomb-shaped ozone-decomposingmaterial 27.

In other words, since effluent ozone should not be ignored insemiconductor production factories, the ozone is decomposed to a levelequal to or less than that in natural air through the two-stepdecomposition.

In FIG. 5, reference numeral 4 denotes an air outlet of the clean room 1and reference numeral 5 denotes a fan.

Example 6

FIG. 6 represents an another embodiment in which the apparatus of thepresent invention is applied to purify air for feeding an air knife in aclean room 1 of class 100 of the same type as that in Example 5.

As shown in FIG. 6, the apparatus of this example is different from thatof Example 5 (FIG. 5) in that decomposition section (B) for decomposingthe microparticles of organic compounds with a photocatalyst is followedby another microparticle-producing section (A) using UV irradiation.

In other words, the apparatus of this example is arranged in accordancewith the scheme of A→B→A→C.

Owing to this arrangement, the apparatus of this example can subjectacidic gas to microparticle-producing step twice, thus facilitating theremoval thereof in the trapping section. The air 14 in the clean room isfreed of gaseous contaminants more efficiently than in Example 1.Gaseous contaminants are decreased to one fiftieth or less of theconcentration at the inlet.

In FIG. 6, reference numerals identical to those in FIG. 5 denote thesame elements as those in FIG. 5.

Example 7

FIG. 7 represents another embodiment in which the apparatus of thepresent invention is applied to purify air for feeding an air knife in aclean room 1 of class 100 of the same type as that in Example 5.

As shown in FIG. 7, the apparatus is arranged in accordance with thescheme of A→C→A→B. This type of apparatus is effective for treating airhaving a high concentration of acidic or basic gases, hence suitable foruse in acid washing or alkali treating operation in a clean room.

As shown in FIG. 7, gaseous contaminants are transformed intomicroparticles (A), and then the resultant microparticles of particulatematter is trapped and removed (C), thus enabling to decrease theconcentrations of acidic and basic gases.

Then, gaseous contaminants such as organic compounds are transformedinto microparticles again (A), and then the resultant microparticlesoriginating in organic compounds and basic gas are decomposed with aphotocatalyst (B). Reference numeral 25 b denotes a HEPA filter whichcan act in case of emergency for collecting an eventually presentmicroparticles (particulate matter) in the upper stream

Since acidic or basic gases at high concentration can be trapped andremoved prior to reaching the photocatalytically decomposing section(B), possible sources (e.g. acidic gas) of a catalytic poison to thephotocatalyst 32 can be removed, thus allowing for a long-term stableoperation.

By virtue of the apparatus of the present invention, organic compoundsare decomposed to a level of not more than 0.1 ppm in terms ofnon-methane organic compounds taken as an indicator. Operations asstated above makes gaseous contaminants increase to a level of 100 to500 ppb in terms of SO₂ concentration, which can be removed to a levelof not more than 1 ppb owing to the apparatus of the present invention.

In FIG. 7, reference numerals identical to those in FIG. 5 denote thesame elements as those in FIG. 5.

Example 8

FIG. 8 represents another embodiment in which the apparatus of thepresent invention is applied to purify air for feeding an air knife in aclean room 1 of class 100 of the same type as that in Example 5.

As shown in FIG. 8, the apparatus is arranged in accordance with thescheme of A→C→B. This type of apparatus is effective for treating airhaving a high concentration of acidic or basic gases.

As shown in FIG. 8, gaseous contaminants are transformed intomicroparticles (A), and then the resultant microparticles of particulatematter is trapped and removed (C), thereby enabling to decrease theconcentrations of acidic and basic gases.

Then, microparticulate matter originating in organic compounds and basicgas are decomposed with a photocatalyst (B). Reference numeral 25 bdenotes a HEPA filter which can act in case of emergency for collectingan eventually present microparticles (particulate matter) in the upperstream.

Since acidic or basic gases at high concentration can be trapped andremoved prior to reaching the photocatalytically decomposing section(B), possible sources (e.g. acidic gas) of a catalytic poison to thephotocatalyst 32 can be removed, thus allowing for a long-term stableoperation.

By virtue of the apparatus of the present invention, organic compoundsare decomposed to a level of not more than 0.1 ppm in terms ofnon-methane organic compounds taken as an indicator. Operations asstated above makes gaseous contaminants increase to a level of 100 to500 ppb in terms of S0 ₂ concentration, which can be decreased to reacha level of not more than 1 ppb owing to the apparatus of the presentinvention.

In FIG. 8, reference numerals identical to those in FIG. 5 denote thesame element as those in FIG. 5.

Example 9

FIG. 9 represents another embodiment in which the apparatus of thepresent invention is applied to purify air for feeding an air knife in aclean room 1 of class 100 of the same type as that in Example 5.

As shown in FIG. 9, the apparatus 12 of the present invention of Example5 is modified in such a way that an integral section (B+C) is formed byintegrating the decomposition section (B) for photocatalyticallydecomposing the microparticles of organic compounds and basic gas withthe removal section (C) for trapping and removing acidic and basic gaseswith photoelectrons.

In other words, the apparatus is arranged in accordance with the schemeof A→B+C.

The air 14 in the clean room 1 is irradiated with an UV lamp(low-pressure mercury lamp) 15 so as to transform organic compounds 30 aas well as acidic and basic gases 30 b into particulate matter 16 (A).

An integral section composed of the photocatalytical decompositionsection (B) and the trapping section (C) for trapping microparticles ofparticulate matter comprises an UV lamp 31 (19), aphotoelectron-emitting material 20 coated on the surface of the UV lamp31 (19), an electrode 21, a photocatalyst 32 coated on the surface ofthe electrode 21 and a trapping material 10 which is placed downstreamto the UV lamp 31 for trapping charged particulate matter. The UV lamp31 (19) is a bactericidal lamp and has a dual function of irradiatingthe photoelectron-emitting material 20 (for emission of photoelectrons)and the photocatalyst 32 (for induction of photocatalysis). Thephotoelectron-emitting material 20 is coated over the UV lamp 31 (19).By forming an electrical field of 50 V/cm between thephotoelectron-emitting material 20 and the electrode 21, photoelectronscan be efficiently emitted towards the electrode 21. Among theparticulate matter 16, particles originating in the acidic and basicgases 30 b can be electrically charged with the photoelectrons emittedfrom the photoelectron-emitting material 20. The resultant chargedparticulate matter is trapped and removed with the trapping material 10for trapping the charged particulate matter.

Among the particulate matter 16, microparticles originating in organiccompounds 30 a and basic gas can deposit on the surface of thephotocatalyst 32 which had been activated by irradiation of the UV lamp31, and subsequently they can be decomposed and removed throughphotocatalysis (B).

In a method for removing microparticles in an electrical field thatcomprises: irradiating a photoelectron-emitting material withultraviolet radiation so as to induce the emission of photoelectrons;supplying microparticles with an electrical charge by thephotoelectrons; and trapping the charged microparticles while forcingthem to move by the action of the electrical field, the effects of thephotocatalyst can be further improved by incorporating the photocatalystinto an electrode that forms the electrical field (Japanese PatentApplication No. Hei-8-231290).

Example 10

Purification of the air in a wafer stocker (wafer receiving stocker) 36in a clean room of class 100 in a semiconductor production factory isdescribed with reference to a basic arrangement shown in FIG. 10.

Organic compounds 30 a and coexistent acidic and basic gases 30 bincluding SO₂ and NH₃ contained in the air present in the stocker 36 aretreated by a purification unit 12 of the present invention disposed inthe stocker 36. The purification unit 12 comprises amicroparticle-producing section 8 (A) for producing microparticles oforganic compounds and coexistent gaseous contaminants by UV irradiation,a decomposition section 26 (B) for decomposing the microparticles oforganic compounds with a photocatalyst, an ozone-decomposing section 27and a trapping section (C) composed of sections 9, 10 for trapping themicroparticles of particulate matter with photoelectrons.

The air 14 in the stocker 36 containing organic compounds 30 a as wellas coexistent gases 30 b including acidic gas such as SO₂ and basic gassuch as NH₃ are treated in the unit 12 of the present invention tobecome clean air 28. This air 28 is extremely clean and is superior toclass 1 and has a concentration of organic compounds below 0.01 ppm andconcentrations of acidic and basic gas respectively of below 1 ppm.

By placing the wafers 34 held on a wafer case 35 into the wafer stocker36 of this embodiment and exposing the wafers to the extremely clean airas mentioned above, the wafers can be maintained without the increase inthe contact angle (no increase in the contact angle occurs on the wafer34 received in the part D of the wafer stocker 36) and the change of theelectrical properties.

This embodiment is arranged in accordance with the scheme below:microparticle-producing section by UV irradiation (A)→decompositionsection for decomposing the microparticles of organic compounds with aphotocatalyst (B)→removal (dust-removal) section for trapping themicroparticles of particulate matter (C).

This embodiment is described in more detail.

By opening the stocker 36 disposed in the clean room 1, the air 14 ofclass 100 in the clean room 1 flows into the stocker 36. This air 14 iscontaminated with organic compounds in a concentration of 0.8 to 1.5 ppmin terms of non-methane hydrocarbons as well as with acidic gas such asSO₂ and basic gas such as NH₃. SO₂ is present in a concentration of 10to 15 ppb and NH₃ is present in a concentration of 30 to 50 ppb.

When organic compounds 30 a in air in the wafer stocker 36 are depositedonto a substrate such as a wafer, the organic compounds 30 a increasethe contact angle. When acidic and basic gases 30 b in air in the waferstocker 36 are deposited onto a substrate, the acidic and basic gases 30b may adversely influence the electrical properties of the wafer. Theseorganic compounds 30 a and acidic and basic gases 30 b can betransformed into particulate matter 16 upon irradiation with an UV lamp(low-pressure mercury lamp) (A).

Among particulate matter 16, particles originating in the organiccompounds 30 a and the basic gas which can be readily adsorbed onto anadsorptive surface such as that of photocatalytic material (TiO₂) maydeposit (be adsorbed) on the surface of the photocatalyst 32 which hasbeen activated by irradiation with an UV lamp 31, and then be decomposedand removed through photocatalysis (B).

On the other hand, among particulate-matter 16, particles originating inacidic gas which cannot be readily decomposed with the photocatalyst 32pass through the photocatalytically decomposing section B into thetrapping and removal section (C) that follows. The section (C) iscomposed of an lamp 19, a photoelectron-emitting material 20, anelectrode 21 (a charging part for supplying particulate matter with anelectrical charge) and a material for trapping the charged particulatematter 10 with photoelectrons.

In the section (C) for trapping and removing with photoelectrons, inflowparticulate matter 16 can be electrically charged with photoelectrons(not shown) emitted from the photoelectron-emitting material 20 and theresultant charged particulate matter can be trapped and removed with atrapping material 10. The photoelectron-emitting material 20 is coatedon the UV lamp 19. By forming an electrical field of 50 V/cm between thephotoelectron-emitting material 20 and the electrode 21, photoelectronscan be efficiently emitted.

As stated above, particles 16 originating in organic compounds 30 a andbasic gas can deposit on the surface of the photocatalyst 32 (carriedand fixed on the surfaces of surrounding walls and glass rods) that hadirradiated with the UV lamp 31 (bactericidal lamp) and can be decomposedand removed efficiently through photocatalysis. In consequence, organiccompounds can be decomposed to a concentration of not more than 0.2 ppm,more preferably 0.1 ppm in terms of non-methane organic compounds takenas an indicator.

In other words, organic compounds having a high molecular weight andactive organic compounds that are responsible for the increase in thecontact angle are decomposed to organic compounds having a low molecularweight that can avoid increasing the contact angle or carbon dioxide andwater, depending upon the types of the compounds.

Further, acidic and basic gases present in the air prevailing in thewafer stocker 36 are decreased to the one tenth of the initialconcentration. In terms of SO₂ and NH₃, the concentration of each of SO₂and NH₃ in this stocker is decreased to a value of not more than 1 ppb.

Microparticles present in the air 14 of class 100 in the wafer stocker36 are, together with particulate matter 16 originating in gaseouscontaminants as stated above, can be trapped and removed in the section(C) for trapping and removing with photoelectrons in the same manner asdescribed above.

The air 28 which was obtained by using the purification apparatus 12 ofthe present invention is freed from organic compounds and coexistentgaseous contaminants, resulting in an extremely clean air superior toclass 1.

The air 14 in the wafer stocker 34, while flowing in the form of the airstreams 28, 33 a, 33 b and 14, can be effectively treated duringsuccessive passages through the microparticle-producing section (A), thedecomposition section for decomposing the microparticles of organiccompounds with a photocatalyst (B) and the section for trapping andremoving the particulate matter in the form of microparticles (C). Theseair streams 28, 33 a, 33 b and 14 are generated as the result of thedifference between the temperatures above and below of the purificationunit 12, this difference being caused by irradiation with the UV lamp15, 31 and 19. In the manner as described above, the part D storing thewafer 34 set in a wafer case 35 can be effectively cleaned.

By exposing the surfaces of the wafer 34 to an extremely clean air asstated above, the wafer surfaces can escape from contamination. Inconsequence, the contact angle on a wafer can avoid increasing. Thisavoidance of increase in the contact angle has the effects ofreinforcing the adhesion of a thin film formed on a wafer substrate (AirPurification, Vol. 33, No. 1, 16-21, 1995).

Ozone generated by irradiation with the UV lamp 15 for producingmicroparticles can be decomposed and removed to a level of not more than0.01 ppm by the action of the photocatalyst 32 as stated above and ahoneycomb-shaped ozone-decomposing material 27.

Though this example describes about the case in which gas is air, thepresent invention can of course be applied as well to the cases in whichgas is another gas such as nitrogen or argon which is contaminated withorganic compounds and gaseous contaminants including acidic and basicgases. The present invention can be applied not only under atmosphericpressure, but also under an increased or decreased pressure.

In FIG. 10, reference numerals identical to those in FIG. 5 denote thesame elements as those in FIG. 5.

Example 11

FIG. 11 shows a purification apparatus 12 of a type different from thatof Example 10 shown in FIG. 10.

As shown in FIG. 11, an ozone-decomposing material 27 and aphotocatalytically decomposing section 26 are disposed in the reversedorder so that the ozone-decomposing material 27 comes next to theparticle-producing section 8 (A). The arrangement shown in FIG. 10 canexhibit the effects comparable to those of the apparatus shown in FIG.10. In FIG. 11, reference numerals identical to those in FIG. 10 denotethe same elements as those in FIG. 10.

Example 12

FIG. 12 represents a purification unit 12 of a type different from thatof Example 10 shown in FIG. 10.

The unit 12 shown in FIG. 12 is modified in such a way that thedecomposition section (B) for decomposing the microparticles of organiccompounds with a photocatalyst is integrated with the section (C) fortrapping and removing the acidic and basic gases in the form ofmicroparticles with photoelectrons.

In other words, the unit is arranged in accordance with the scheme ofA→B+C.

Organic compounds 30 a as well as acidic and basic gases 30 b aretransformed into particulate matter 16 (A) by irradiation with an UVlamp (low-pressure mercury lamp) 15.

An integral section composed of photocatalytically decomposing section(B) and the section (C) for trapping the microparticles of particulatematter comprise an UV lamp 31 (19), a photoelectron-emitting material20, an electrode 21 carrying a photocatalyst 32 and a material 10 fortrapping the charged particulate matter.

Among the particulate matter 16, particles originating in organiccompounds 30 a and basic gas can deposit (be adsorbed) on the surface ofthe photocatalyst which had been activated by irradiation with the UVlamp 31, and hence can be decomposed and removed through photocatalysis(B).

In a method for removing microparticles by irradiating aphotoelectron-emitting material with an ultraviolet radiation in anelectrical field, incorporation of a photocatalyst into an electrodeforming the electrical field may improve the action of the photocatalyst(Japanese Patent Application No. Hei-8-231290).

On the other hand, among the particulate matter 16, particlesoriginating in acidic and basic gases 30 b can be electrically chargedwith photoelectrons (not shown) emitted from the photoelectron-emittingmaterial 20. The resultant charged particulate matter can be removed bythe material 10 for trapping the charged particulate matter. Thephotoelectron-emitting material 20 is coated onto the UV lamp 19. Byforming an electrical field of 50 V/cm between thephotoelectron-emitting material 20 and the electrode 35, photoelectronscan be efficiently emitted. Charged particulate matter can be removed bythe trapping material 10.

In this manner, the air 14 in the wafer stocker 36 is treated with theunit 12 of the present invention, resulting in the clean air 28. Thisair 28 is an extremely clean air superior to class 1 in which organiccompounds have been decomposed and removed to be a level of not morethan 0.01 ppm and the acidic and basic gases 30 such as S0 ₂ and NH₃that had been present in the air 14 in the clean room have been removedto be a level of not more than 1 ppb.

In FIG. 12, reference numerals identical to those in FIG. 10 denote thesame element as those in FIG. 10.

Example 13

FIGS. 13 and 14 represents a purification unit 12 of a type differentfrom that of Example 10 shown in FIG. 10.

The unit 12 shown in the FIGS. 13 and 14 is modified in such a way thatthe microparticle-producing section 8 (A) is integrated with thedecomposition section 26 (B) for decomposing the microparticles oforganic compounds by means of a photocatalyst.

In other words, the apparatus of this example is arranged in accordancewith the scheme of A+B→C.

The photoelectron-emitting material 20 is formed on the surface ofwall-forming materials by Au plating.

In FIG. 13, the photocatalyst 32 is coated on the surface of a wall. Thephotocatalyst 32 is activated to perform photocatalytic action byirradiation with the UV lamp 15 for producing particles.

In FIG. 14, the photocatalyst 32 is provided on or near the surface ofan ozone-decomposing material 27. The photocatalyst 32 is activated toperform photocatalytic action by irradiation with the U lamp 15 forproducing particles.

In FIGS. 13 and 14, reference numerals identical to those in FIG. 10denote the same element as those in FIG. 10.

Example 14

A sample of gas as specified below was charged into a stocker arrangedas shown in FIG. 10, a wafer was placed therein, and then the contactangle on the wafer, the concentrations of non-methane organic compounds,SO₂, HCl, NH₃ and ozone present in the stocker were measured.

Experimental Conditions

(1) sample gas: air of class 10 in a semiconductor production factory;

concentration of non-methane organic compound: 0.8 to 1.2 ppm;

SO₂ concentration: 10 to 30 ppb;

HCl concentration: 3 to 5 ppb;

NH₃ concentration: 10 to 20 ppb

(2) stocker volume: 80 liters;

(3) UV lamp for producing particles: low-pressure mercury lamp (184 nm);

(4) photocatalyst: titanium dioxide carried on a glass fiber plate bysol-gel process;

(5) light source: bactericidal lamp (254 nm) (for irradiating aphotocatalyst and for emitting photoelectrons);

(6) photoelectron-emitting material: an Au layer having a thickness of 8nm plated on the above-mentioned bactericidal lamp

(7) electrode materials and electrical field:

for charging: Cu—Zn, 50 V/cm;

for trapping particulate matter: Cu—Zn, 500 V/cm

(8) ozone-decomposing material: honeycomb-shaped composite oxidecatalyst, MnO₂/ZrO—C;

(9) wafer: a highly pure 5 inch silicon wafer was cut into pieces of 1cm×8 cm and placed in the stocker;

(10) pretreatment of the wafer: washing with a detergent and alcohol ona clean bench in the clean room, followed by UV/O₃ cleaning;

(11) measurement of the contact angle: the contact angle was measuredwith a CA-D type contact angle feeler manufactured by Kyowa KaimenKayak, Inc.;

(12) concentration of non-methane organic compound: was measured by gaschromatography (GC) method;

(13) SO₂ concentration: was measured by solution conductivity method;

(14) HCl concentration: was measured by absorption liquid method;

(15) NH₃ concentration: was measured by chemiluminescent (LCD) methodand liquid absorption method;

(16) ozone concentration: was measured with a chemiluminescent ozonedensitometer;

(12) opening of the stocker: the stocker was disposed in a clean zone(class 10) of a semiconductor production factory and the open-closecycles were repeated 6 times per day.

Results:

(1) FIG. 15 shows the contact angle on a wafer as the function of thenumber of days during which the wafer was stored in the stocker.

In FIG. 15, the symbol ◯ represents the values obtained according to thepresent invention and the symbol ● represents the values obtained from a(comparative) test in which the wafer was exposed to the air of class 10in a clean room. The symbol ↓ means that the obtained value is below thelimit of detection.

(2) Concentrations of non-methane organic compound, SO₂, HCl, NH₃ andozone present in the stocker are shown in the Table 2. TABLE 2 storageorganic period compound SO₂ conc. HCl conc. NH₃ conc. ozone conc. (days)conc. (ppm) (ppb) (ppb) (ppb) (ppb) 1 <0.1 <1 <0.5 <1 <0.1 2 <0.1 <1<0.5 <1 <0.1 10 <0.1 <1 <0.5 <1 <0.1

Besides, the concentrations of organic compounds and SO₂ (after 1 day ofstorage) were determined in a similar manner, except that the removalsection (C) for trapping with photoelectrons was omitted. The obtainedvalues were respectively <0.1 ppm and 10 to 25 ppb.

The concentration of the microparticles in the stocker was undetectable(after 30 minutes, 1 day and 10 days of storage) with a particlecounter. Therefore, the air in the stocker had a purity superior toclass 1.

Switching the UV lamp for producing particles generates the ozoneconcentration of 15 to 20 ppm.

(3) In a stocker without the unit of the present invention, a wafer wasstored for 2 or 7 days, Then, the wafer was taken out of the stocker,heated to desorb the organic compounds deposited thereon. Analysis ofthe wafer by gas chromatography/mass spectrometry (GC/MS) showed thepresence of phthalic esters such as DOP. The unit of the presentinvention was disposed in the stocker and a wafer was treated andanalyzed in the same way. Phthalic esters such as DOP was undetectable.

In the embodiment described below, after a gas is cleared of acidicand/or basic gases, organic compounds and residual basic gas can bedecomposed with a photocatalyst. Acidic gases which can be mentioned arenitrogen oxides (NOx), nitrogen oxide ion, sulfur oxides (SOx), sulfuroxide ion, hydrogen chloride and hydrogen fluoride. Basic gases whichcan be mentioned are ammonia and amines.

Example 15

FIG. 18 represents a wafer stocker 71. This wafer stocker is disposed ina clean room of class 10,000 in a semiconductor production factory.

1.0 to 1.5 ppm of non-methane hydrocarbons, 30 to 40 ppb of SOx and 60to 80 ppb of NH₃ are present in the clean room. A wafer is placed in thestocker 71 so as to be protected against these gaseous contaminants.

In other words, a wafer carrier 3 holding the wafer 72 can be introducedinto or withdrawn from the stocker 71 by opening the door of the stocker71. At every opening of the door, contaminants present in the clean roompenetrate into the stocker 71. These organic compounds 74 may cause thecontact angle to increase. SOx may bring about defective insulation ofan oxide film. NH₃ may bring about defective resolution of a wafer.

The apparatus of the present invention is provided with an ion exchangefiber 76 and a decomposition section situated above the ion exchangefiber. The ion exchange fiber is preferably in the form of a knittedfilter.

The decomposition section consists of an UV lamp 77, a catalyticallyactive material coated in the form of a film on the surface of the UVlamp 77 and a light-screening material 80. Catalytically active materialis preferably TiO₂. The light-screening material 80 is provided forprotecting the wafer 72 from irradiation with slight UV leakage of theUV lamp 77.

The UV lamp 77 irradiates the thin film of TiO₂ coated on the surfacethereof, allowing the TiO₂ to perform the photocatalytic action fordecomposing organic compounds effectively.

UV irradiation causes, a slight temperature difference between the aboveand the below of the photocatalyst 78. This temperature difference givesrise to air convection in the stocker 71, thus generating the airstreams 79 a, 79 b and 79 c circulating in the stocker. As a result, airpasses through the ion exchange fiber 76 and comes into contact with thephotocatalyst 78 that follows.

First, acidic and basic gases are removed with the ion exchange fiber76. Acidic gases can be removed with an anionic ion exchange fiber,whereas basic gases can be removed with a cationic ion exchange fiber.Organic compounds 74 present in the air, having passed through the ionexchange fiber, are decomposed with the photocatalyst.

The air in the clean room contains organic compounds in a concentrationof 1.0 to 1.5 ppm, SOx in a concentration of 30 to 40 ppb and NH₃ in aconcentration of 60 to 80 ppb. These contaminants penetrate into thestocker 71 on opening of the stocker 71. By virtue of the apparatus ofthe present invention, organic compounds can be decomposed to a level ofnot more than 0.1 ppm in terms of non-methane hydrocarbons taken as anindicator. At the same time, SOx and NH₃ can be trapped with the ionexchange fiber 76, thereby removed to a level of not more than 1 ppb. Asthe result, the wafer 72 stored in the stocker 71 can be maintained withno increase in the contact angle, no defective insulation and nodefective resolution.

The present invention can be applied to not only air in a clean room asusual but also various gases, e.g. N₂ and Ar, as well.

In FIG. 18, it is preferred to dispose between the ion exchange fiber 76and the UV lamp 77 an additional section having an UV lamp for producingmicroparticles of organic compounds and organosilicon compounds.Alternatively, the UV lamp 77 and the photocatalyst 78 shown in FIG. 18can be replaced by the microparticle-producing section 8 and thedecomposition section 26 shown in FIG. 2.

Example 16

FIG. 19 represents another embodiment of a wafer stocker 71. This waferstocker is disposed in a clean room of class 10,000 of a semiconductorproduction factory in a manner similar to Example 15.

In example 16, by opening the stocker 71, organic compounds 74, SOx andNH₃ 75 as well as particulate matter 81 (microparticles) including ionssuch as NO₃ ⁻, NO₂ ⁻ and SO₄ ⁻ penetrate into the stocker 71.

Thus, unlike the apparatus of example 15 shown in FIG. 18, the section Cis disposed for charging and trapping the microparticles 81 withphotoelectrons. More precisely, the section C consists of an UV lamp 82,a photoelectron-emitting material 83 coated on the surface of the UVlamp 82, an electrode 84 surrounding the UV lamp 82 for emittingphotoelectrons and a trapping material 85 disposed downstream to the UVlamp for trapping charged microparticles.

The photoelectron-emitting material 83 of this example is coated on thesurface of the UV lamp 82 so as to form an integrated device (Laid OpenJapanese Patent Application No. Hei-4-243540). The electrode 84 servesto generate an electrical field (photoelectron-emitting material (−) andelectrode (+)) so that the photoelectron-emitting material 83 may emitphotoelectrons efficiently.

In the stocker 71, microparticles 11 are carried by air streams 79 a to79 c that are circulating in the apparatus of the present invention.First, SOx and NH₃ are trapped and removed with an ion exchange fiber76. Second, particulate matter including NO₃ ⁻, NO₂ ⁻ and SO₄ ²⁻ aretrapped and removed in the section C after being electrically chargedwith photoelectrons. In the section C of this embodiment, microparticlesare electrically charged with the photoelectrons that had been emittedfrom the photoelectron-emitting material 83 upon irradiation with the UVlamp 82, to become charged microparticles, which can be in turn trappedand removed with the trapping material 85 placed downstream Organiccompounds 74 present in the air that had passed through the trappingmaterial 85 can be decomposed with the photocatalyst 78.

As stated above, purified air that has been cleared of acidic and basicgases, ion-containing particulate matter as well as gaseous organiccompounds is present in the space B shown in FIG. 19. In other words,SOx and NH₃ are decreased to a level of not more than 1 ppb and organiccompounds are decreased to a level of not more than 0.1 ppm, thuscreating a purified space superior to class 1.

In FIG. 19, reference numerals identical to those in FIG. 18 denote thesame element as those in FIG. 18.

In FIG. 19, it is preferred to dispose an additional section between thetrapping material 85 and the UV lamp 77 for producing microparticles oforganic compounds and organosilicon compounds. Alternatively, the UVlamp 77 and the photocatalyst 78 shown in FIG. 19 may be replaced by themicroparticle-producing section 8 and the decomposition section 26 shownin FIG. 2.

Example 17

FIG. 20 represents an embodiment other than that shown in FIG. 19.

In the embodiment shown in FIG. 20, the ion exchange fiber 76 and thetrapping section 85 shown in FIG. 19 are formed as an integral sectionfor trapping and removing acidic and basic gases. In FIG. 20, referencenumerals identical to those in FIG. 19 denote the same element as thosein FIG. 19.

As shown in FIG. 20, the lamp 82 is placed upstream to the ion exchangefiber 76, which in turn is placed upstream to the trapping section 85.The ion exchange fiber 76 and the trapping section 85 are integrated. Aphotoelectron-emitting material 83 is coated on the wall opposite to theUV lamp 82, not on the surface of the UV lamp 82. A photocatalyst 78 iscoated also on the wall opposite to the UV lamp 77.

In this example, particulate matter 81 including NO₃ ⁻, NO₂ ⁻ and SO₄ ²⁻are first trapped and removed in the trapping section (C) by beingcharged electrically with photoelectrons. Second, acidic and basic gasesincluding SOx and NH₃ are trapped and removed with the ion exchangefiber 76. And finally, organic compounds 74 are decomposed with thephotocatalyst 78 and removed.

As stated above, purified air that is free from acidic and basic gases,ion-containing particulate matter as well as gaseous contaminants(organic compounds) is present in the space B shown in FIG. 20 (SOx andNH₃: not more than 1 ppb; organic compounds: not more than 0.1 ppm),thus creating an extremely purified space superior to class 1. A waferplaced in this space B can be protected from an increase in the contactangle, defective insulation of oxide film, defective resolution, as wellas circuit breakage and shortage.

In FIG. 20, it is preferred to dispose between the trapping material 85and the UV lamp 77 an additional section having an U lamp for producingmicroparticles of organic compounds and organosilicon compounds.Alternatively, the UV lamp 77 and the photocatalyst 78 in FIG. 20 can bereplaced by the microparticle-producing section 8 and the decompositionsection 26 shown in FIG. 2.

Example 18

FIG. 21 represents an embodiment of a type other than that shown in FIG.19.

In the embodiment shown in FIG. 21, ion exchange fiber is omitted, andhence acidic and basic gases are removed in a trapping section C only bybeing charged electrically by photoelectrons.

This embodiment is suitable for use in purifying a gas of the type thatcontains acidic and basic gases such as SOx and NHx at a relatively lowconcentration and ion-containing microparticles such as NO₃ ⁻, NO₂ ⁻ andSO₄ ³⁻ at a relatively high concentration.

An UV lamp 82 irradiates a photoelectron-emitting material 83, allowingthe latter to emit photoelectrons. The photoelectrons can supply theparticles with an electrical charge and the resultant charged particlescan be removed in a trapping section 85. Then, a photocatalyst 78irradiated with an UV lamp 77 can oxidatively decompose the organiccompounds and basic gases.

In FIG. 21, it is preferred to dispose between the trapping material 85and the UV lamp 77 an additional section having an UV lamp for producingmicroparticles of organic compounds and organosilicon compounds.Alternatively, the UV lamp 77 and the photocatalyst 78 shown in FIG. 21may be replaced by the microparticle-producing section 8 and thedecomposition section 26 shown in FIG. 2.

In FIG. 21, reference numerals identical to those in FIGS. 18 to 20denote the same element as those in FIGS. 18 to 20.

Example 19

FIG. 22 represents an embodiment of the purification apparatus 70 of thepresent invention applied to purify air for feeding an air knife device89. The purification apparatus 70 and the air knife device 89 aredisposed in a clean room of class 10,000 of a semiconductor productionfactory.

The purification apparatus of the present invention comprisessuccessively downstream an ion exchange fiber in the form of a filter76, a dust-removing filter 87 a, a decomposition section and adust-removing filter 87 b. The decomposition section is composed of anaxially extending UV lamp 77 and a photocatalyst 78. The photocatalyst78 is coated on the inner surface of a housing. A glass rod is placed inthe axial direction of the decomposition section in parallel to the UVlamp and has the surface which is also coated with a catalyticallyactive material.

Non-methane hydrocarbons are present in the concentration of 1.1 to 1.3ppm in the clean room of class 10,000. SOx, NOx and NH₃ are present inthe concentration of 40 ppb, 30 ppb and 150 ppb (average concentration),respectively.

From the air 16 in the clean room, acidic and basic gases are removedfirst by trapping action of the ion exchange fiber 76 until theconcentrations of SOx and NH₃ taken as contamination indicators aredecreased each to a level of not more than 1 ppb. Then, microparticlespresent in the air in the clean room are removed with the dust-removingfilter 87 a.

Then, with the aid of the photocatalyst (TiO₂) 78 which had beenactivated by UV radiation emitted from the UV lamp 77, organic compoundsare decomposed until the concentration of non-methane hydrocarbons takenas contamination indicator are decreased to a level of not more than 0.1ppm.

Filters suitable for use as the dust-removing filter 87 b (dust-removingsection) are those that in case of emergency can efficiently trap themicroparticles which eventually flow out at or near the ion exchangefiber and the organic compound-decomposing section. An ULPA filter isused in this embodiment.

An extremely pure air 88 that is superior to class 1 (NOx, SOx, NH₃:below 1 ppb; organic compounds: below 0.1 ppm) and is free from acidicand basic gases as well as particulate matter and gaseous contaminants(organic compounds) can be obtained in a manner as described above. Theobtained extremely pure air 88 is fed to the air knife device 89.

In FIG. 22, it is preferred to dispose an additional section between thefilter 87 a and the decomposition section composed of the UV lamp 77 andthe photocatalyst 78, the additional section being amicroparticle-producing section having an UV lamp for producingmicroparticles of organic compounds and organosilicon compounds.Alternatively, the UV lamp 77 and the photocatalyst 78 shown in FIG. 22can be replaced by the microparticle-producing section 8 and thedecomposition section 26 shown in FIG. 2.

Example 20

A sample of gas as specified below was charged into a stocker as shownin FIG. 18 and the open-close cycles were repeated 10 times per day. Thecontact angle on the wafer stored in the stocker was measured under along-term continuous operation. The concentrations of non-methanehydrocarbons and SOx in the air present in the stocker were measured andthe hydrocarbons deposited on the wafer in the stocker were identified.

-   Sample gas: air of class 10 in a clean room    -   concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm    -   Sox concentration: 40 to 60 ppb;-   Stocker volume: 80 liters;-   Light source: low-pressure mercury lamp (with peaks at 184 nm and    254 nm);-   Photocatalyst: TiO₂;-   The way of carrying photocatalyst on the light source: TiO₂ was    coated onto the surface of the mercury lamp to a thickness of 50 nm    by sol-gel method;-   Means for trapping and removing acidic and basic gases: an ion    exchange fiber (anion-type) or a fibrous active charcoal;-   Fabrication of the ion exchange fiber: anion exchange fiber: graft    polymerization of fibrous polypropylene was carried out by exposing    to an electron beam of 20 Mrad under nitrogen and then immersing in    a solution consisting of hydroxystyrene monomer and isoprene.    Quaternary amination of the reaction product afforded an anion    exchange filter;-   Measurement of the contact angle: a method for measuring the contact    angle of a water drop (manufactured by Kyowa Kaimen Kagaku Inc.,    CA-DT type);-   Concentration of non-methane hydrocarbons in the stocker: was    measured by gas chromatography (GC) method;-   Concentration of SOx in the stocker: was measured by solution    conductivity method;-   Identification of the hydrocarbons adsorbed on the wafer: GC/MS    method;-   Wafer stored in the stocker: a highly pure 5-inch silicon wafer was    cut into pieces of 1 cm×8 cm and pretreated as stated below prior to    introduction into the stocker;-   Pretreatment of the wafer: washing with a detergent and alcohol on a    clean bench in the clean room, followed by UV/O₃ cleaning. The wafer    was exposed to UV radiation under the conditions allowing O₃    evolution.    Results:    (1) The Contact Angle on the Wafer

FIG. 23 shows the contact angle (in degrees) as the function of thenumber of days when an ion exchange fiber was employed. In FIG. 23, thesymbol-◯-represents the combination of an ion exchange fiber and aphotocatalyst (present invention), the symbol-●-represents a controlwithout ion exchange fiber and the symbol-▴-represents a control withoutphotocatalyst.

The control without photocatalyst showed the contact angle of 20 degreesat 20 hours after and 40 degrees at 60 hours after, thus no preventiveeffect was observed on the increase in the contact angle. The measuredvalues as long as the 350th day are shown in FIG. 23. When an ionexchange fiber was disposed at the inlet in the manner as shown in FIG.22, the apparatus could maintain a high performance withoutdeterioration with time.

The control applying a fibrous active charcoal in place of an ionexchange fiber for trapping and removing acidic and basic gases showedthe results similar to those shown in FIG. 23 and exhibited an increasein the contact angle of below 2 degrees after 300-day operation.

(2) Organic Compounds in the Stocker

The concentrations of non-methane hydrocarbons and SOx as well as theidentification of the hydrocarbons adsorbed on the wafer (presence orabsence of the identified components) are shown in the table 3. Theresults obtained by using a control having an ion exchange fiber withoutphotocatalyst or a control having a photocatalyst without ion exchangefiber are also shown in the table 3. TABLE 3 storage hydrocarbon periodconc. in the deposition on SOx conc. in (days) conditions air (ppm) thewafer the air (ppb) 1 invention <0.1 absent <1 no photocatalyst 1.2˜1.5present <1 no ion exchange <0.1 absent 5˜7 fiber 10 invention <0.1absent <1 no photocatalyst 1.2˜1.5 present <1 no ion exchange <0.1absent 5˜7 fiber 100 invention <0.1 absent <1 no photocatalyst 1.2˜1.5present <1 no ion exchange <0.1 absent 10˜15 fiber 300 invention <0.1absent <1 no photocatalyst 1.2˜1.5 present <1 no ion exchange 0.5˜0.8absent 15˜20 fiber

In table 3, the hydrocarbon deposited on the wafer was phthalic estersuch as DOP.

Example 21

The same apparatus as that used in the example 21 was used, except thata photocatalyst was employed and an ion exchange fiber was omitted. Thenumber of days required for a 5 degree increase in the contact angle wasobserved as the function of the SOx concentration by using a sample gasadjusted to a SOx concentration of 1 to 50 ppb. Other experimentalconditions were same as those used in Example 20.

-   Sample gas: air of class 10 in a clean room-   Concentration of non-methane hydrocarbon: 1.2 to 1.5 ppm-   SOx concentration: 1 to 50 ppb

The concentration of SOx was appropriately adjusted by regulating thepassing speed of the gas through the ion exchange fiber for removing apart of SOx as desired.

Results

FIG. 24 represents the number of days required for a 5 degree increasein the contact angle versus the SOx concentration in the sample gas. Itwas obvious that the less was the concentration of sulfur oxides in asample gas, the more was the number of days required for increase in thecontact angle. In other words, the air was kept clean for longer.

FIG. 25 is a total ion chromatogram obtained by gas chromatography/massspectrography of organic compounds in the air. The axis of abscissa xrepresents the mass of an ion and the axis of ordinate y represents arelative strength. 30 liters of sample air was flown into an adsorbent(TENAX-GR) at a rate of 0.5 liter/min. The adsorbent was heated in adevice for concentration and introduction (manufactured by CHROMPACKInc., CP4010 model) so as to desorb the adsorbed gas, which was cooledwith liquid nitrogen, concentrated and measured in a gaschromatography/mass spectrometry device (manufactured by ShimazuSeisakusho Inc., QP-1100EX model).

(a) is a graph showing the results of the air of class 10,000 in a cleanroom. Non-methane hydrocarbons are present in a concentration of 1.1 to1.3 ppm and SOx, NOx and NH₃ are present in a concentration of 40 ppb,30 ppb and 150 ppb (average concentration) respectively. Each peakrepresents the presence of an organic compound.

(b) is a graph showing the results of the air of class 10,000 in a cleanroom after treated by the apparatus of the present invention. It isobvious that the amount of organic compounds was drastically decreased.

The present invention, by producing microparticles of contaminants andconcentrating them locally, can decompose the contaminants with aphotocatalyst effectively, even when the contaminants are present at lowconcentration. Contaminants such as organic compounds in the air, evenwhen they are present at low concentration, can be efficientlydecomposed because they are made into microparticles and brought intocontact with a photocatalyst in the concentrated form. As a result, thedecomposition rate of organic compounds was improved. The photocatalystis effective in decomposing oxidizable compounds including gaseousorganic compounds such as phthalic ester, gaseous organosiliconcompounds such as siloxane and basic gas such as ammonia. The presentinvention is particularly advantageous in view of the fact that gaseousorganic compounds and gaseous organosilicon compounds are difficult toremove by photoelectronical charging or with a filter or an ion exchangefiber.

When a removal section utilizing a filter, an adsorbent, a photoelectronand an ion exchange resin is provided, other gaseous contaminants whichare present together with organic compounds, including acidic gases suchas SO₂, NOx, HCl, HF and the like as well as basic gases such NH₃,amines and the like can also be removed. In other words, a wide varietyof contaminants ranging from gas to particles can be removed.

In addition, when acidic and basic gases are removed prior to thetreatment with a photocatalyst, adverse effects on the photocatalyst bythe acidic and basic gases can be avoided. In consequence, thephotocatalyst can act reliably over a long period, thus allowinglong-term operation.

By way of an illustrative application, the present invention wasdescribed mainly with reference to a clean room of semiconductorproduction factories.

The present invention are also suitable to other applications as setforth below:

-   (1) Air purification for public welfare, for example, purification    of air in offices, buildings, houses, hospitals, hotels and the    like;-   (2) Purification of exhaust gases originating in various industries    including sewer drains and waste disposal sites; purification of    industrial atmosphere; as well as purification of exhaust gases    originating in underground parking areas and tunnel ventilators; and-   (3) Purification of air and gases including nitrogen, oxygen and the    like for use in gas production units for feeding clean rooms, clean    booths, clean tunnels, clean benches, safety cabinets, bioclean    boxes, sterile rooms, path boxes, precious article stockers,    transportation spaces, interfaces, air curtains, air knives, drying    sections, production lines and the like in pioneer industries such    as semiconductor industries, electronic industries, pharmaceutical    industries, food industries, agricultural and forestry industries,    medical industries, precision machinery industries and the like.

Public welfare was mentioned in consideration of the fact that gaseouscontaminants present in the air may adversely affect human healthproducing so-called sick building syndrome.

1-20. (canceled)
 21. A method for purifying a gas containing acontaminant and a solid particle, comprising: removing said solidparticle from said gas; irradiating said gas with an ultraviolet rayand/or a radiation ray, thereby producing microparticles of saidcontaminant; contacting said microparticles of said contaminant with aphotocatalyst which comprises mainly TiO₂; and irradiating saidphotocatalyst with a light, thereby decomposing said microparticles ofsaid contaminant in contact with said photocatalyst and thereby removingnon-methane organic compounds as a part of said contaminant to a levelof not more than 0.2 ppm.
 22. The method of claim 21, wherein saidcontaminant comprises at least one species selected from the groupconsisting of an organic compound, an organosilicon compound and a basicgas; with the proviso, that the contaminant is not an alkane.
 23. Themethod of claim 21, wherein said gas is irradiated with said ultravioletray having a wavelength of not more than 260 nm, thereby producing saidmicroparticles of said contaminant.
 24. The method of claim 21, whereinsaid gas contains water in a concentration of not less than 1 ppb orgaseous oxygen in a concentration of not less than 1 ppb.
 25. The methodof claim 24, wherein said gas contains water in a concentration of notless than 100 ppb or gaseous oxygen in a concentration of not less than100 ppb.
 26. The method of claim 24, wherein said gaseous oxygen ispresent in said gas in a concentration of not less than 100 ppb, whereinsaid gaseous oxygen is transformed into ozone during said irradiation ofsaid gas with the ultraviolet ray and/or a radiation ray; and whereinsaid method further comprises decomposing said ozone.
 27. The method ofclaim 21, further comprising: removing said contaminant with at leastone member selected from the group consisting of a filter, an adsorbent,an ion exchanger and a photocatalyst, preceding or following saidproduction of microparticles.
 28. The method of claim 27, wherein saidcontaminant comprises an acidic compound or a basic compound.
 29. Themethod of claim 27, wherein said contaminant comprises at least onespecies selected from the group consisting of nitrogen oxide (NO_(x)),nitrogen oxide ion, nitrogen sulfide (SO_(x)), nitrogen sulfide ion,hydrogen chloride, hydrogen fluoride, ammonia and amines.
 30. The methodof claim 27, wherein said contaminant is removed after said productionof microparticles, followed by said decomposing of said microparticlesof said contaminant in contact with said photocatalyst.
 31. The methodof claim 27, wherein said microparticles of said contaminant aredecomposed before said removing of said contaminant.
 32. The method ofclaim 27, wherein said photocatalyst is used so as to supply saidcontaminant with an electrical charge and trap the resultant chargedcontaminant.
 33. The method of claim 21, wherein said photocatalystcomprises a matrix and a catalytically active component comprisingmainly TiO₂ carried on said matrix, said catalytically active componentbeing in the form of a particle.
 34. The method of claim 33, whereinsaid matrix has a shape selected from the group consisting of ahoneycomb structure provided with at least one partition defining atleast 2 through-holes, a bar body and a wall member, and saidcatalytically active component consists of TiO₂.
 35. The method of claim21, wherein said photocatalyst consists essentially of TiO₂.
 36. Themethod of claim 21, wherein said photocatalyst consists of TiO₂.